Step-by-step Solution:

We analyze each number:\[\] 1. 21 = (3 \(\times\) 7) (multiple of 7) \[\] 2. 36 = (6 \(\times\) 6) (multiple of 7]) \[\] 3. 49 = (7 \(\times\) 7) (perfect square) \[\] 4. 56 = (7 \(\times\) 8) (multiple of 7]) \[\] The correct choice is: 36\[\] \[\] Correct Option: (b)