The question requires us to identify the molecular formula of a molecule formed between phosphorus (P) and chlorine (Cl).
The first step to solve this question is write the electronic configuration of both atoms involved (P and Cl) and, from the number of valence electrons, identify how many bonds each one should make.
P has atomic number 15, thus its electronic configuration is: 1s2 2s2 2p6 3s2 3p3
Cl has atomic number 17, thus its electronic configuration is: 1s2 2s2 2p6 3s2 3p5
Note that P has 5 valence electrons (3s2 and 3p3) while Cl has 7 valence electrons (3s2 3p5).
For P, we should expect 3 bonds to other atoms in order to reach the octet, while for Cl, we should expect 1 bond to another atom to achieve the octet.
With that in mind, we can say that each Cl will bond to one P, and we need 3 Cl. Therefore, the molecular formular is PCl3 (the last option).I