Find an Algebraic Expression Equivalent to the Given Expression $\cos \left( \arctan \frac { \sqrt { 5 } } { u } \right)$

Multiple Choice

Find an algebraic expression equivalent to the given expression.
- $\cos \left( \arctan \frac { \sqrt { 5 } } { u } \right)$

A) $\frac { \sqrt { 5 } } { u }$
B) $\frac { \sqrt { 5 u ^ { 2 } + 25 } } { 5 }$
C) $\frac { u \sqrt { u ^ { 2 } + 5 } } { u ^ { 2 } + 5 }$
D) $\frac { \sqrt { \mathrm { u } ^ { 2 } + 5 } } { 5 }$