What Is the Angle Between the Unit Vector $\vec { u }$

Multiple Choice

What is the angle between the unit vector $\vec { u }$ and $\nabla f ( a , b )$ if $f _ { a } ( a , b ) = - 0.5 \| \nabla f ( a , b ) \|$ ?

A) $5 \pi / 6$
B) $2 \pi / 3$
C) $\pi / 3$
D) $\pi / 6$