Relatively prime means having no factors in common.
I will start by checking if they have the factor "3" in common: it's easy to check: the sum of its digit needs to be divisible by 3:
a. 72 And 115
7+2=9
1+1+5=7
not true that both are divisible by 3
b. 120 And 835
1+2=3
8+3+5=16
not true that both are divisible by 3
but, they're both divisible by 5! (0 or 5 at the end)
not relatively prime
c. 201 And 735
2+1=3
7+3+5=15
both are divisible by 3!
d. 303 And 1041
3+3=6
1+4+1=6
both divisible by 3
the answer is a)
