A muscle cramp is most likely due to a lack of what? Actin, myosin, atp, or adp?

when lactic acid builds up it causes muscles to hurt.  lactic acid occurs during fermentation .  fermentation yields a lot less ATP than aerobic respiration.  I would say the answer is ATP.
High tide is right in that lack of Oxygen will cause fermentation of lactic acid and lactic acid will build up.  However,  you did not ask about oxygen.  She asked due to actin, myosin, ATP, or ADP, so I gave her the most reasonable answer in that during anaerobic respiration which lactic acid forms(causing the muscles to cramp), only yields 2 ATP's per glucose.  Aerobic respiration will give you about 38 ATP per glucose and no lactic acid formation, hence no cramping.  
So I maintain that a muscle cramp is most likely due to lack of production of ATP!

lack of OXYGEN for aerobic respiration to occur.
when aerobic respiration fail to occur, anaerobic respiration will take place and incomplete oxidation of glucose will happen that produce LACTIC ACID and less ATP is generated.
the accumulation of lactic acid can cause cramp to muscle, especially during vigorous activity!

What is said above is true, but technically it is a lack of oxygen which prevents cells from undergoing the respiratory phase of producing energy.

Muscle cramps are caused since the muscles do not receive enough oxygen in time and are forced to increase fermentation producing the lactic acid.

atp.