Factor the Expression Over the Set of Complex Numbers $9 a ^ { 2 } + 16$

Multiple Choice

Factor the expression over the set of complex numbers $9 a ^ { 2 } + 16$

A) $\quad ( - 3 a + 4 i ) ( - 3 a + 4 i )$
B) $\quad ( 3 a + 4 i ) ( 3 a - 4 i )$
C) $\quad ( 3 a + 4 ) ( 3 a - 4 )$
D) $\quad ( 3 + 4 i ) ( 3 - 4 i )$
E) $\quad ( 3 a + 4 i ) ( 3 a + 4 i )$