Answer
C
Work Step by Step
Find the greatest common factor. Find the factors of each term and multiply the factors that are common to all the terms.
$21n^3=7\times\underline3\times\underline n\times\underline n\times n$
$18n^2=6\times\underline3\times\underline n\times\underline n$
GCF=$3\times n\times n=3n^2$
