Aphosphorus atom and a chlorine atom are likely to form a covalent bond because both elements:
are nonmetals
have low electronegativity values
have equal "pull" on shared electrons
lack valence electrons

Option (a) is the correct answer.

Explanation:

When there occurs sharing of electrons between the two combining atoms then the bond formed is known as a covalent bond. Whereas when there occurs transfer of electrons between the combining atoms then the bond formed is known as ionic bond.

An ionic bond is formed between a metal and a non-metal.

Both phosphorous and chlorine belongs to third period and both of them are non-metals.

Atomic number of P is 15 and its electronic distribution is 2, 8, 5. Atomic number of Cl is 17 and its electronic distribution is 2, 8, 7.

Therefore, in order to attain stability both P and Cl will combine with each other by sharing electrons and hence, they will form a covalent bond.

Thus, we can conclude that a phosphorus atom and a chlorine atom are likely to form a covalent bond because both elements are non-metals.