Which joins amino acids together?
a peptide bonds
b hydrogen bonds
c van der waals forces
d ionic bonds

A peptide bonds

Amino acids are joined together when an enzyme called peptidyl transferase catalyzes a bond between the carboxyl group (COOH) of one amino acid and the amino group (NH2) of the second amino acid to form a peptide bond in a reaction called dehydration synthesis because a water molecule is lost in the process. The CO of one amino acid binds to the NH of the other to form this bond thus a larger molecule is synthesized.
This molecule can again be broken into individual amino acids by simply adding a molecule of water to the peptide in the presence of an acid e.g  hydrochloric acid  or protease enzyme e.g pancreatic protease. The acid provides the extra hydrogen atom needed to form water.  This process or reaction is called hydrolysis.

option a is correct i.e.

only sodium ions can pass through the membrane.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.