The correct answer is:
7ab³
Explanation:
The greatest common factor is the largest thing that each term is divisible by.
The coefficients of our terms are 42, 35 and 42. Using the prime factorization for each,
42 = 6*7
6 = 2*3
42 = 2*3*7
35 = 5*7
The greatest common factor is 7.
For the variables, each term has at least a¹ and at least b³. This means that ab³ is part of the GCF; this gives us a GCF of 7ab³.