The question involves determining which statements about multiple access communication techniques are correct. Let's evaluate each statement:

1. Statement (a): "In CDMA, the entire bandwidth can be used all the time."

Explanation: Code Division Multiple Access (CDMA) allows many users to share the same frequency bandwidth simultaneously. Each user is assigned a unique code, and the entire bandwidth is available to all users at any time. Hence, this statement is correct.

1. Statement (b): "TDMA is a technique which does not allow for spectrum re-use i.e. utilizing same spectrum in different time slots."

Explanation: Time Division Multiple Access (TDMA) allows multiple users to share the same frequency channel by dividing the signal into different time slots. Each user transmits in rapid succession, one after the other, each using its own time slot. Therefore, TDMA does allow spectrum re-use across time slots. This statement is incorrect.

1. Statement (c): "TDMA has primary disadvantage of susceptibility for station cross-linking."

Explanation: The primary disadvantage of TDMA is not typically described as susceptibility for station cross-linking but rather issues like synchronization between different users, as each user needs precision in timing. Hence, this statement is misleading and incorrect.

1. Statement (d): "FDMA is the simplest & most basic format Multiple Access Communications."

Explanation: Frequency Division Multiple Access (FDMA) is one of the earliest forms of multiple access used in communication systems. It allocates individual frequency bands to different users. Being one of the oldest and simplest forms, this statement is correct.

Given the analysis above, statements (a) and (d) are correct. Therefore, the correct answer is:

(a) & (d) are correct.