C

To determine the change in speed, we analyze the components of the initial velocity 'u'. Assuming 'u' is the initial speed at an angle θ, we can resolve this speed into two components:

• The horizontal component is given by: $u_x = u \cos(\theta)$
• The vertical component is given by: $u_y = u \sin(\theta)$

The change in speed refers to the vertical component, as it is most affected by gravity in projectile motion. The vertical component changes as the object moves up and down, specifically, at its highest point the speed becomes zero in the vertical direction. Thus, the change in speed associated with the vertical motion is given by $u \sin(\theta)$ as it represents the component reducing due to gravity (until it reaches the peak). Therefore, the correct answer is the vertical component:
Therefore, the change in speed is: $u \sin(\theta)$.
Hence, the correct option is C.