n k product is 400

IMO E.

nk = 400

a) n + k > 0 ( not true.. n and k can both -20 and their sum can be less than 0)

b) n<>K ( Not true... n and k can both be 20 or -20)
c) n or k is a multiple of 10 ( 400 can be written as 4*5*4*5 (=20*20) .. Which can be written as 16*25. So n or k may not be a multiple of 10)
d) If n is even , then k is odd (Not always.. if n is 20, k should be 20 which is even)
e) If n is odd , then k is even (Must be true. If n is 25(or 5), then k must be 16(80) which is even. ) Bcoz odd * even = even.

Strategy is POE and the process is clearly explained by Mani_mba.

imo:E