Covalent bonds tend to make atoms more stable by them

fill up their innermost electron shell.
fill up their outermost electron shell.
empty out their innermost electron shell.
empty out their outermost electron shell.

fill up their outermost electron shell.

Explanation: Covalent bonds are formed by sharing of electrons between two non metals.

The sharing of electrons take place in order for the elements to attain stable configuration by filling their outermost shells.

For example: is formed by sharing of electrons between two fluorine atoms.

Fluorine has an atomic number of 9 has got 9 electrons.

In order to get stable, fluorine must share one electron with another fluorine atom as the outermost electron shell will be filled on sharing of electrons.

the part of the wave he is travelling on is the crest