In Caesar Cipher algorithm:
If we have to Decrypt the message, Decrease(count backwards) the value given key(15 in this question) times.
If we have to Encrypt the message, Increase(count forward) the value given key times.
Here, we have to Decrypt WTAAD:
W=23, T=20, A=1, A=1, D=4 and key = 15.
So, as already mentioned above we have decrypt by 15.
W = 23-15 = 8 = H
T = 20-15 = 5 = E
A = 1-15 = -14 = -14+26 = 12 = L // You can also simply get it by counting 15 times backwards from A, like z,y,x,w.......
A = 1-15 = -14 = -14+26 = 12 = L
D = 4-15 = -11 = -11+26 = 15 = O
Hence, Option (B) is correct.