(b) 300%
By Joule's law, $H = I^2Rt$. If current doubles, new heat $H' = (2I)^2Rt = 4I^2Rt = 4H$.
Percentage change $= \frac{4H - H}{H} \times 100 = 300\%$.
Source: Electricity, Section 11.7
Students often confuse "percentage change" with "times increased." Heat becomes 4 times the original, but the increase is 3H, giving 300% change — not 400%. Always apply the formula: % change = [(new − old)/old] × 100.