A man generated his email password a 4-digit number ‘abcd’. Since he didn’t log into his account for quite some time, he forgot the password. To recollect the original password again, he writes 9 separate probable passwords:

2186, 5127, 6924, 4351, 5916, 8253, 4521, 6384, 8517 In each of the nine passwords, at least one of the digits

a, b,

c, and d occurs in the correct position.

What is the value of d?

(a) 1

(b) 3

(c) 4

(d) 6