Solution:

(57242)9×7×5×3×1 = (57242)945

The unit digit of the resultant number only depends on the unit digit of the given number 57242, i.e. 2. Now,

21 = 2
22 = 4
23 = 8
24 = 16 (unit digit 6)
25 = 32 (unit digit 2)
.
.
.
And so on.

Exponent of any number ending in 2 will produce a number that will end in 2, 4, 8, or 6.
Now, 945 = 944 + 1
944 is divisible by 4.
So, the last digit of (57242)945 will be the same as that of (57242)1, which is 2. Hence, option (a) is correct.