Which is more stronger acid ? Explain with the H+ dissociation

... Explain with the H+ dissociation

So why don't you give that a try?

Start by looking both compounds up in a table of pK_a values.

Which dissociates more completely. As BabcockHall said, check the pk_a values, it should be clear.

Ya it is clear with pka values but how formic acid can be stabilize more than C6H5COOH after H+ dissociation

I might offer a guess that it has something to do with solvation. How could you test that hypothesis?

It has to do with stability. You should ask yourself whether the phenyl ring is electron donating or withdrawing in this context.

My understanding of Felix Carroll's table of gas-phase acidities (Table 7.4, p. 406) suggests to me that benzoic acid is actually stronger than formic acid in the gas phase.