Answer
$C$
Work Step by Step
List the prime factors of each term. Identify the factors common to all terms.
$21n^{3}=(3)\cdot 7\cdot(n\cdot n)\cdot n$
$18n^{2}=2\cdot(3)\cdot 3\cdot(n\cdot n)$
GCF= $(3)\cdot(n\cdot n)$, or $3n^{2}$.
Answer: C.
