(iii) M + N

In a sliding window ARQ scheme with sender window size N and receiver window size M, the minimum number of distinct sequence numbers required is N + M.

Why: Sequence numbers must be large enough that the sender's window and the receiver's window can never simultaneously refer to overlapping sequence numbers in a way that causes ambiguity between old and new frames. If the total range of usable sequence numbers were smaller than N + M, it would become possible for a frame from a previous window cycle to be mistaken for a frame in the current cycle, since the sender's and receiver's windows could "wrap around" and overlap incorrectly.

This is a well-known general result (a classic GATE-style question) that applies to any generic sliding window ARQ with independent sender/receiver window sizes — note that this differs from the more commonly quoted Go-Back-N-specific rule (sequence numbers ≥ N+1) because that assumes a receiver window of size 1 (M=1), giving N+M = N+1, consistent with the general formula.