We know :
Efficiency = N / (1 + 2a) where a = tp / tf
Hence ,
N / (1 + 2a) = 1/2
==> N = ceil((1 + 2a) / 2)
==> N = ceil(( 1 + (1000/16) ) / 2)
==> N = 32
Hence senders window size = 32
Hence total sequence numbers needed = 2 * 32 = 64
Hence no of bits needed for sequence no = log2 64
= 6
Hence 6 is the correct answer..