(a) Most of the energy in the leg muscles of Abhinav for running the marathon is provided by aerobic respiration (which breaks down food like glucose with oxygen to produce large amount of energy).
(b) The little extra energy in the leg muscles for running very, very fast is provided by the process of anaerobic respiration (which breaks down food like glucose without oxygen to produce small amount of energy).
(c ) Lactic acid.
(d) The lactic acid gets accumulated in the leg muscles of Abhinav because during very fast running, oxygen gets used faster in the muscles than can be supplied by the blood. So, anaerobic respiration (without oxygen) takes place in muscle cells causing partial oxidation of glucose food to form lactic acid (and releasing some energy). This lactic acid gets accumulated in leg muscles.
(e) Abhinav should take a hot water bath or a massage. Hot water bath and massage improve the circulation of blood. Due to improved blood flow, the supply of oxygen to the leg muscles increases. This oxygen breaks down lactic acid accumulated in leg muscles and gives relief from cramp.