Which of the following best defines the greatest common factor (GCF) for a list of algebraic terms?

Find the GCF of the integers 84 and 126 using prime factorization.

Factor completely by extracting the GCF: $14x^3-21x^2+35x$

Factor completely by grouping: $x^3+4x^2-x-4$

Which factoring strategy is most efficient for $x^4-16$, and what is the complete factorization?