答案：C

解析：

Choice C is correct. Tony's first goal is that his total fundraiser sales from 15-dollar boxes of candy and 20-dollar t-shirts will be more than $300. Tony's fundraiser sales can be calculated by (price per T-shirt)*(number of T-shirts sold) + (price per box of candy)*(number of boxes of candy sold). If Tony meets this goal, this value will be greater than $300. This situation can be expressed with the following inequality: 15c+20t＞300.

Tony's second goal is that he will sell at least as many T-shirts as boxes of candy. Therefore, the second inequality should show that the number T-shirts sold, t, is greater than or equal to the number of boxes of candy sold, c. This situation can be expressed with the following inequality: t≥c.

Therefore, the following system of inequalities best models the situation: 15c+20t≤300 t≥c.