Long-day plants normally do not flower during the winters.Horticulturalists can trigger the process of flowering in a long-day plant during winters by

Explanation

Long-day plants require a certain length of daylight to trigger flowering. By artificially extending the day length with additional light exposure during the night, horticulturalists can induce flowering in these plants even during the shorter days of winter. Option B is correct because it involves increasing the exposure to light, simulating longer days. Option A is incorrect as warmth does not directly affect photoperiodism. Option C is incorrect because reducing the light period with a black cloth simulates shorter days, which is unsuitable for long-day plants. Option D is incorrect as covering with transparent plastic does not alter the light exposure significantly.