Direct Tunnel in Rel 7 is the connection between…a.MSC - GGSNb.GGSN - RNCc.MSC - SGSNd.SGSN - RNC

The Direct Tunnel in Rel 7 is the connection between SGSN - RNC. So, the correct answer is d. SGSN - RNC.

Here's a step-by-step explanation:

1. In the context of mobile communications, Rel 7 refers to Release 7, a set of specifications for 3G networks issued by the 3rd Generation Partnership Project (3GPP).

2. The Direct Tunnel is a feature introduced in Release 7 to improve data transfer efficiency in 3G networks.

3. The Direct Tunnel establishes a direct path between the Serving GPRS Support Node (SGSN) and the Radio Network Controller (RNC), bypassing the Gateway GPRS Support Node (GGSN).

4. Therefore, the Direct Tunnel in Rel 7 is the connection between the SGSN and the RNC.