What is the greatest common factor of 42a5b3, 35a3b4, and 42ab4?

7ab3

6a4b
42a5b4
77a8b7

The answer is option A
       
If we decompose the three options in three prime factors, we will get:
       
42a^5b^3: 2 · 3 · 7 · a · a · a · a · a · b · b · b
   
35a^3b^4: 5 · 7 · a · a · a · b · b · b · b
   
42ab^4: 2 · 3 · 7 · a · b · b · b · b
       
We see that numer 7 is a prime factor for all, the minimum exponent for letter a is 1, and the minimum exponet of b is 3, so the answer is 7ab^3.