Explanation
The command show xc 1 displays a list of cross-connections, including A-end, Z-end, frequency, ID number, OCH trail name, and administrative/operational states. A cross-connection is a logical connection between two ports on an optical network element that allows the transmission of an optical signal from one port to another.
The command can be used to view the configuration and status of the cross-connections on a node or a network. The other options are incorrect because they either describe a different command or provide incorrect information. References: Nokia Optical Diagnostics and Troubleshooting Course, OAM and Diagnostics Guide

Explanation
The recommended order for the troubleshooting steps is B, as follows:
* Retrieve the channel power trace. This step is useful to identify the affected channel and its power level, as well as to check if there are any fluctuations or anomalies in the power trace that could indicate a channel absent issue1.
* Retrieve the cross-connection (XC) details and see what Wave Keys should be present. This step is necessary to verify which Wave Keys are expected to be present on the node, card, and port based on the XC configuration2. Wave Keys are unique identifiers for wavelength tracking that are encoded by Optical Transponders (OTs) into each service wavelength direction3.
* Go to the suspected troubled node / card / port and look at Wave Keys (in / out). This step is helpful to compare the observed Wave Keys with the expected Wave Keys, and to locate the source of the problem. If a Wave Key is missing or mismatched, it means that there is a channel absent issue on that node, card, or port4.
* Check observed Wave Keys against expected Wave Keys. This step is the final solution to resolve the issue and restore the normal operation of the node. The observed Wave Keys should match the expected Wave Keys based on the XC configuration. If not, the XC configuration should be corrected or the faulty node, card, or port should be replaced5. References : Nokia Optical Diagnostics and Troubleshooting Course | Nokia, Optical User Guide - Nokia, Alcatel-Lucent 1830 PSS-8 and PSS-16 Photonic Service Switch

Explanation
The command that should be used to configure the 10GbE client interface 1/7/C1 with a GFP-F encapsulation mode is config interface 1/7/C1 encmode 10client gfp-f. This command will set the encapsulation mode of the interface to GFP-F, which is a frame-mapped generic framing procedure that encapsulates Ethernet frames with a GFP header. The command also specifies that the interface is a 10GbE client interface, which means that it supports 10 Gigabit Ethernet LAN signals. The other commands are incorrect because they either have invalid syntax or use incorrect parameters for the interface or the encapsulation mode. References: Nokia Optical Diagnostics and Troubleshooting Course, OAM and Diagnostics Guide

Explanation
The payload type setting is an attribute that is recorded within the client payload and can be set automatically or manually. The payload type setting indicates the type of client signal that is carried by the OTN frame, such as Ethernet, Fibre Channel, or SDH/SONET. The payload type setting can be used for service identification and performance monitoring purposes. The payload type setting can be set automatically by the ML-Series card, which can detect the client signal type and encode it in the payload header. Alternatively, the payload type setting can be set manually by the user using the command config interface <interface> encmode
<encmode> payloadtype <payloadtype>, where <interface> is the client interface name, <encmode> is the encapsulation mode, such as GFP-F or BMP, and <payloadtype> is the client signal type, such as 10GE LAN or FC-1200. The other options are incorrect because they either state that the payload type setting is recorded within the OTN overhead, which is not true, or that it must be entered manually, which is not necessary. References: Nokia Optical Diagnostics and Troubleshooting Course, OAM and Diagnostics Guide

Explanation
The command show interface am2125a 1/6/lineout detail is used to retrieve the total output power level of the AM2125A amplifier module. This command displays detailed information about the lineout interface, including the current optical power, wavelength, and status. The total output power level is shown as Output Power (dBm) in the output of this command1. References : Nokia Optical Diagnostics and Troubleshooting Course | Nokia