Find the Composite Function $f ( g ( x ) )$

Multiple Choice

Find the composite function $f ( g ( x ) )$ $f ( u ) = \frac { 1 } { u } , g ( x ) = x + 3$

A)
$f ( g ( x ) ) = \frac { 1 } { x } + 3$
B)
$f ( g ( x ) ) = x + 3 + \frac { 1 } { x }$
C)
$f ( g ( x ) ) = 1 + \frac { 3 } { x }$
D)
$f ( g ( x ) ) = \frac { 1 } { x + 3 }$