The substrate must fit perfectly with an enzyme's this is known as the and key fit.

The substrate must fit perfectly with an enzyme's active site this in known as the lock and key fit.

Explanation:

The lock and key model was given  by the Emil Fisher.  The lock is represent an enzyme which is complementary to its substrate and substrate is called key.

when an enzyme is used to catalyzed the reaction, there are active sites on the enzymes through which substrate bind. The active site have specific geometry which is complementary to the substrate so substrate perfectly fit into it.

It is resemble with the lock and key. As there is a specific key for a specific lock this model is just like this.

c is the answer because when something is left after something evaporates that shows the chemical change.

the number of valence electrons for each molecule or ion is shown beneath the structure. remember hydrogen will not have more than two electrons. this hydrogen is part of a covalent bond (sharing two electrons). a double bond here would cause hydrogen to share four electrons with phosphorus.