a. H and Cl can combine to form ionic bonds.
Ionic bonds are formed when a metal atom loses electrons to become a positively charged ion and a nonmetal atom gains electrons to become a negatively charged ion. The electrostatic attraction between the two ions holds them together.
In option a., H is a nonmetal and Cl is a halogen. Halogens are nonmetals that readily gain electrons to form anions. Therefore, H can donate an electron to Cl, forming an H+ ion and a Cl- ion and an ionic bond is formed between them.
In option b., H and N are both nonmetals, they do not have the ability to lose or gain electrons to form ionic compounds.
In option c., S and Sr are both nonmetals and metals respectively, they do not have the ability to lose or gain electrons to form ionic compounds.
In option d., Xe is a noble gas and F is a halogen, they do not have the ability to lose or gain electrons to form ionic compounds.