A prime number is a positive integer greater than 1 
which is not divisible by any number except itself
and 1.

All the factors are 3, 3^2, 3^3, 3^4 and 3^5.   

3 is not divisible by any positive integer besides itself, 3, and 
1 and it is greater than 1, so it IS a prime factor of 3^5

3^2, which is 9, is divisible by the positive integer 3, and 3 is
a positive integer other than 9. So it is NOT a prime factor of 3^5.

3^3, which is 27, is divisible by the positive integer 3, and 3 is
a positive integer other than 27. So it is NOT a prime factor of 3^5.

3^4, which is 81, is divisible by the positive integer 3, and 3 is
a positive integer other than 81. So it is NOT a prime factor of 3^5.

3^5, which is 243, is divisible by the positive integer 3, and 3 is
a positive integer other than 243. So it is NOT a prime factor of 3^5.

Therefore the largest prime factor of 3^5 is 3.  [Can you see, from the
pattern above that 3 is the largest prime factor of even
3*1000000000 ?]

Can you also see, by the same reasoning, that the largest prime factor 
of 5^3 is 5? 

answer (a) says 3 is greater than 5.  We know that's incorrect.
answer (b) says 5 is greater than 3.  We know that's correct.
answer (c) says 3 is equal to 5.  We know that's incorrect.
answer (d) say which is larger cannot be determined. We know that's
  incorrect because we can determine that 5 is greater than 3.

The only correct answer is (b) 

Edwin