What does the sign pictured above mean?

The answer is:

A. Repeat the preceding music.

If you'd like to know more about the

The sign that you see there is the repeat sign. This means that you will repeat that section. If there is no other repeat sign on the musical score, then that means you have to start from the very beginning. If there are more than one, you should see another repeat sign before the one you encountered, but facing the other way. That sign will indicate where you will start the repeat.

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