Step 1: Recall why rural and urban norms differ.
Poverty lines in India were first fixed using minimum calorie needs. Rural work involves more physical labour on average, so its calorie norm was set higher, at 2400 kcal per day.
Step 2: Find the urban figure.
Urban lifestyles were assumed to need fewer calories, and the fixed urban norm is 2100 kcal per person per day.
Step 3: Rule out the other choices.
1900 kcal was never an official norm. 2400 kcal already belongs to rural areas as stated in the question. 2800 kcal is even higher than the rural figure, which does not make sense for urban areas with lower assumed calorie needs.
Step 4: Conclude.
The correct urban calorie limit is 2100 kcal per person per day.