The network element responsible for control part in 4G is….a.RNCb.MMEc.PGWd.SGW

The network element responsible for control part in 4G is MME (Mobility Management Entity).

Here's a step by step explanation:

1. In a 4G network, the control plane is responsible for setting up and managing network connections. This includes functions like authentication, security, and mobility management.

2. The Mobility Management Entity (MME) is the key control-node for the LTE network. It is responsible for idle mode User Equipment (UE) tracking and paging procedure including retransmissions.

3. The MME is involved in the bearer activation/deactivation process and is also responsible for choosing the Serving Gateway (SGW) for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation.

4. Therefore, the answer to the question "The network element responsible for control part in 4G is…" is MME (Mobility Management Entity).