Simplify Assume That Variables Represent Positive Real Numbers $( 3 x ) ^ { 4 / 9 }$

Multiple Choice

Simplify. Assume that variables represent positive real numbers.
- $( 3 x ) ^ { 4 / 9 }$

A) $\sqrt [ 4 ] { 3 x ^ { 9 } }$
B) $\sqrt [ 9 ] { 3 x ^ { 4 } }$
C) $\sqrt [ 9 ] { 81 x ^ { 4 } }$
D) $3 \sqrt [ 9 ] { x ^ { 4 } }$