This is an Assertion–Reason question, so both statements must first be checked individually for truth and then evaluated for whether one explains the other.

Checking Assertion (A): Dry HCl gas does not show acidic behaviour. This is indeed true. In its pure gaseous form, HCl exists as discrete, covalently bonded molecules (H–Cl). There are no free H⁺ ions available, and acidity (in the Arrhenius sense) specifically requires the release of H⁺ ions (or, more accurately, hydronium ions, H₃O⁺) that can react with indicators, metals, carbonates, or bases. Dry HCl gas will not turn dry blue litmus paper red, will not react with active metals to liberate hydrogen, and will not react with dry marble — because none of these tests actually detect the HCl molecule itself, they detect the H⁺/H₃O⁺ ion.

Checking Reason (R): HCl produces H⁺ ions only in the presence of water. This is also true. HCl is a polar covalent molecule that ionises when dissolved in water through the reaction: HCl + H₂O → H₃O⁺ + Cl⁻. Water molecules stabilise the separated ions through hydration (ion–dipole interactions), which supplies the energy needed to break the H–Cl bond heterolytically. Without water (or another ionising/protic solvent), this bond-breaking and ion stabilisation cannot occur, so HCl remains as neutral, non-ionised molecules with no acidic character.

Linking A and R: Since the very reason dry HCl fails to behave as an acid is that it cannot generate H⁺ ions without water, R directly and completely explains why A is true. There is a clear cause-and-effect relationship: no water → no ionisation → no H⁺ ions → no acidic behaviour. Hence both statements are true, and R is the correct explanation of A.

The other answer choices fail because: it is incorrect to say A is false (dry HCl genuinely shows no acidic behaviour); it is incorrect to say R is false (the water-dependence of HCl ionisation is a well-established chemical fact); and although both A and R being individually true is correct, it would be wrong to conclude that R does not explain A, since R is precisely the mechanistic reason behind the observation stated in A.