The number 2 is a factor of 4k, 18k and 12 since
4k = 2*2k
18k = 2*9k
12 = 2*6
It is also the largest common factor of all three expressions.
4 wouldn't work because 4 is not a factor of 18
2k doesn't work, and neither does 4k, since k is not present in 12.