You may need to replace a physical disk because the disk is in warning - predictive failure status. The predictive failure status indicates that the physical disk will soon fail, and should be replaced at the earliest opportunity. The Oracle ASM disks associated with the grid disks on the physical drive are automatically dropped, and an Oracle ASM rebalance relocates the data from the predictively failed disk to other disks.
Note: After the physical disk is replaced, the grid disks and cell disks that existed on the previous disk in that slot are re-created on the new physical disk. If those grid disks were part of an Oracle ASM group, then they are added back to the disk group, and the data is rebalanced on them, based on the disk group redundancy and ASM_POWER_LIMIT parameter.
References:
http://docs.oracle.com/cd/E80920_01/DBMMN/maintaining-exadata-storage-servers.htm#DBMMN21047

Explanation/Reference:
C: Scan on a clustered table can prevent a Smart Scan from occur.
D: Scan on an index-organized table can prevent a Smart Scan from occur.
Note: The Cell physical IO interconnect bytes returned by smart scan metric shows how many bytes of I/O were returned by a smart scan to the database server.
References: https://uhesse.com/2011/01/19/exadata-part-i-smart-scan/

Determine the Amount of Available Space
To increase the size of the disks in a disk group you must either have unallocated disk space available, or you have to reallocate space currently used by a different disk group.
Example: View the space currently used by the disk groups.
SELECT name, total_mb, free_mb, total_mb - free_mb used_mb, round(100*free_mb/total_mb,2) pct_free FROM v$asm_diskgroup ORDER BY 1;

The example above shows that the DATAC1 disk group has only about 15% of free space available while the RECOC1 disk group has about 87% free disk space. The PCT_FREE displayed here is raw free space, not usable free space. Additional space is needed for rebalancing operations.
References:
http://docs.oracle.com/cd/E80920_01/SAGUG/exadata-administering-asm.htm#SAGUG20526

Explanation/Reference:
The cellinit.ora file is host-specific, and contains all database IP addresses that connect to the storage network used by Oracle Exadata Storage Servers. This file must exist for each database that connect to Oracle Exadata Storage Servers.
The cellinit.ora file contains the database IP addresses.
The cellip.ora file contains the storage cell IP addresses.
Both files are located on the database server host.
References: http://docs.oracle.com/cd/E80920_01/SAGUG/exadata-storage-server- configuring.htm#SAGUG20369

Explanation/Reference:
When the objective option is set to basic, the database resource plan maximum utilization limits are not enforced. For stricter plan conformance, and enforcement of maximum utilization limits, the objective option must be set to something other than basic. The supported IORM objectives are auto, low_latency, balanced, and high_throughput.
The objective may also be changed to low_latency in which case the latency for critical I/Os is very good but there is a significant degradation in scan throughput when both workloads are running concurrently.
Incorrect Answers:
B: The objective can be changed to high_throughput which will increase the scan throughput at the cost of critical I/O latency.
C, D: The recommended objective option is auto which allows IORM to continuously monitor the workloads, and select the best mode based on the active workloads currently on the cells. The objective values of auto and balanced have the same behavior.