Which of the following factors is one of two that is usually most responsible for determining population growth

Economic conditions should be one of these factors. When a family has a reliable source of income then they can plan a larger family. Having a large family is expensive and requires providing food, clothing and shelter to the children from when they are born up until they leave home. Also, a larger family requires a large place to house them which is more expensive. Another factor is that in some less well developed countries with either no pensions or inadequate ones, a large family is considered insurance for the elderly to be supported by the kids as they grow older and start earning an income.

Your correct the answer is d