If anyone is following the NPTEL course(Computer Networks and Internet Protocol), the course discusses the concept of BDP(Bandwidth Delay product) that can also be used to calculate the window size.
https://www.youtube.com/watch?time_continue=812&v=bKHRbqwkMkg&feature=emb_title
Given, RTT = 40ms, therefore one side time = 40/2 = 20ms
BDP (Bandwidth Delay Product) = Link Bandwidth * Link Delay
Hence,
BDP = 64kbps * 20ms = 1280 bits
packet size = 32byte = 256 bits,
number_of_packets = 1280/256 = 5
WindowSize = 2*no_of_packets + 1
w = 2*5 + 1 = 11 (we add +1 as the ACK is sent only when the first first segment is received)
Hence, (b) 10 is the correct option (closest option)