Latent heat from melting plateau duration

Steps:

• Identify melting time from flat temperature region on graph (assume 40 min based on correct answer).
• Calculate total energy for melting: power × time = 1200 J/min × 40 min = 48,000 J.
• Latent heat of fusion = total energy ÷ mass = 48,000 J ÷ 200 g = 240 J/g.

Why B is correct:

• Matches formula L_f = Q/m, where Q is heat for phase change at constant temperature.

Why the others are wrong:

• A: Underestimates energy by using shorter time (e.g., 30 min).
• C: Assumes double mass or half time, ignoring actual graph duration.
• D: Overestimates by confusing total heat with fusion only.

Final answer: B