At STP 1g of hydrogen fluoride was found to occupy a volume of 0.56 L. the molecular formula of hydrogen fluoride is

HF

H2F2

(HF)3

(HF)4

Those numbers are subscripts.

As we know, 22.4 litre volume contains = 1 mole

So, 0.56 litre contains = 22.4 / 0.56 = 1/40 moles

So, Molar mass = Given mass / Number of moles

= 40 g

Now consider the given options, if we calculate the molar mass of all options, then option 2 has molar mass equals to 40. So, option (2) is set according to the molar mass. So, molecular formula will be H₂F₂.