This is a direct application of Faraday's First Law of Electrolysis, which states that the mass of a substance deposited (or liberated) at an electrode is directly proportional to the quantity of electric charge passed through the electrolyte.

Mathematically:

• m = Z × Q = Z × I × t
• m = mass deposited
• Z = electrochemical equivalent of the substance (a constant for a given material)
• Q = charge = current × time
• I = current (held constant here)
• t = time for which current flows

Physically, charge is carried by ions; each ion that reaches the electrode adds a fixed amount of matter. Passing charge for twice as long lets twice as many ions arrive, so twice the mass is deposited. With Z and I constant, m ∝ t. Therefore, if the time is doubled, the charge doubles and the mass deposited also doubles.

Why the other options are wrong: the deposit cannot remain unchanged or become zero, because more charge always transfers more material as long as current keeps flowing. It cannot become half either — halving would require the time (or current) to be reduced, whereas here the time is increased. The only relationship consistent with m ∝ t is that doubling the time doubles the amount deposited.