To configure extended discovery to support the MPLS LDP session between the headend and tailend routers, the engineer must configure a targeted neighbor session. This action allows for the establishment of LDP sessions between routers that are not directly connected, which is necessary for setting up MPLS tunnels that span multiple hops in the network.

When router A is the Rendezvous Point (RP), Protocol Independent Multicast - Sparse Mode (PIM-SM) is the appropriate mode to configure. In PIM-SM, multicast traffic is initially sent to the RP by the sources. The receivers express their interest in receiving specific multicast traffic by sending join messages towards the RP. This mode is efficient for networks with sparse multicast groups where not all members participate in every multicast session. Reference: = Implementing and Operating Cisco Service Provider Network Core Technologies course materials.

Cisco MPLS TE (Traffic Engineering) tunnels provide a way to optimize traffic routing within an MPLS network. However, there are certain constraints to be aware of:
QoS-aware tunneling: Unfortunately, Cisco MPLS TE tunnels do not inherently support Quality of Service (QoS) awareness. This means that the tunnels themselves do not directly consider QoS parameters when making routing decisions. QoS policies must be applied separately at the network level, and the TE tunnels do not inherently take QoS into account during path selection.
Tunnels spanning multiple OSPF areas: Contrary to option A, Cisco MPLS TE tunnels can indeed span multiple OSPF areas. The use of OSPF areas does not inherently restrict the creation of MPLS TE tunnels across area boundaries.
ISIS metrics: Option B is incorrect. When using ISIS (Intermediate System to Intermediate System) as the Interior Gateway Protocol (IGP), newer-style metrics (such as wide metrics) are supported. Older-style metrics are not the only option.
Tunnels over IP unnumbered links: Option C is also incorrect. Cisco MPLS TE tunnels can be configured over IP unnumbered links, allowing efficient use of IP addresses.
In summary, the primary constraint is the lack of built-in QoS awareness within MPLS TE tunnels.
Reference:
Implementing an MPLS VPN over TE Tunnels - Cisco Support
Configuring Basic MPLS TE - Cisco NX-OS