The final process in production of sperm, yielding cells with small heads and long tails, is called

Spermiogenesis

The final stage of spermatogenesis or production of sperm is called spermiogenesis. It is the stage where spermatids differentiate into mature, motile spermatozoa. 

This composed of four stages: first is the golgi phase where the spermatids start to form a head and its DNA packaging are condensed. Second, the formation of acrosomal cap in cap phase. Third phase is Acrosomal phase, one of the centrioles of the spermatid elongates and become tail and the fourth phase is maturation phase where the remaining cytoplasm and organelles are removed.

it would be c. xylem.

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products