Which of the following molecules is covalently bonded?

...... The answer is : a. P₂O₅

P and O are non-metals. Covalent bonds are formed between non-metal atoms.

The other three options are ionic compounds. In each of them, ionic bonds are formed between the metal ions and the anion ions.

Bonding.....

Some teachers oversimplify bonding and say that all salts (metal/nonmetal or metal/polyatomic ion) have ionic bonds. According to that P2O5 is the one to pick as having covalent bonds. The question is flawed when it says, "...the following molecules..." because none of the compounds exists as discrete molecules. In fact, there is no P2O5. That is simply the empirical formula for P4O10 which exists as a polymer.

There is no Mg2CrO4. The correct formula is MgCrO4 and it exists as a network (lattice) of magnesium atoms and chromate radicals. The bonding is between Mg and O, which is more ionic than covalent.

There is no such compound as Sn(HCO3)2. Only the alkali metals exist as bicarbonate solids.

Silver sulfate is a network solid consisting of alternating Ag atoms and sulfate radicals. The bonding is between oxygen and silver, but the bonds are more covalent than they are ionic.