Which diagram represents an element that is likely to form covalent bonds?

The second option

Explanation:

The octet rule states that elements tend to bond in such a way that each atom has eight electrons in its valence (outermost) shell, except for the first level which is complete with 2 electrons.

The first picture shows an atom with only 1 electron in its valence shell, the second picture 5 electrons, the third pictures 8 electrons and the fourth picture 2 electrons. The first and last cases are more likely to lose their valence electrons to form bonds than share them into a covalent bond (because they are few). The third case has the octet rule already satisfied, so is not likely to neither lose/receive nor share its electrons. The atom of the second case can share 3 electrons to complete the octet rule.