When an enzyme reacts with only limited, selected substrates, this characteristic is referred to as the enzyme's

When an enzyme reacts with only selected and limited number of substrates, then this characteristic of the enzyme is referred to as its specificity. The enzymes which are specific, catalyzes only selected few reactions. These enzymes are specific to certain bonds or functional groups. Some enzymes have absolute specificity, which means that they catalyze only one reaction.

Hence, the blank can be filled with 'specificity'.

b) the inside picture would look at a very high temperature in a very low ph