Factor Out the Greatest Common Factor $21 x ^ { 4 } - 9 x ^ { 3 } + 15 x ^ { 2 }$

Multiple Choice

Factor out the greatest common factor.
- $21 x ^ { 4 } - 9 x ^ { 3 } + 15 x ^ { 2 }$

A) $3 x ^ { 2 } \left( 7 x ^ { 2 } - 3 x + 5 \right)$
B) $x ^ { 2 } \left( 21 x ^ { 2 } - 9 x + 15 \right)$
C) $3 x \left( 7 x ^ { 3 } - 3 x ^ { 2 } + 5 x \right)$
D) $3 \left( 7 x ^ { 4 } - 3 x ^ { 3 } + 5 x ^ { 2 } \right)$