Simplify the Expression $\sqrt [ 4 ] { 81 a ^ { 4 } }$

Multiple Choice

Simplify the expression. Assume all variables represent positive real numbers.
- $\sqrt [ 4 ] { 81 a ^ { 4 } }$

A) $3 a$
B) $3 \sqrt { a }$
C) $3 a ^ { 2 }$
D) $81 a$