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