In a skeletal muscle fibre, the splitting of a molecule of ATP at an ATPase binding site on a myosin ...

I think it is D. actin.

Check out this animation:

http://bcs.whfreeman.com/thelifewire/content/chp47/4702001.html

In a skeletal muscle fibre, the splitting of a molecule of ATP at an ATPase binding site on a myosin head group increases the affinity of the head group’s other binding site for
A. thick filaments.
B. Ca++ ions.
C. troponin
D. actin.
E. ATP.

Yep, I agree with Bio-man...

The cycle starts with myosin (in the absence of ATP) tightly bound to actin. ATP binding dissociates the myosin-actin complex and the hydrolysis of ATP then induces a conformational change in myosin. This change affects the neck region of myosin that binds the light chains, which acts as a lever arm to displace the myosin head by about 5 nm. The products of hydrolysis (ADP and Pi) remain bound to the myosin head, which is said to be in the “cocked” position. The myosin head then rebinds at a new position on the actin filament, resulting in the release of ADP and Pi and triggering the “power stroke,” in which the myosin head returns to its initial conformation, thereby sliding the actin filament toward the M line of the sarcomere.