【正确答案】 C

【答案解析】[理由] 本空格应置入To prepare(准备)，形成不定式短语，置于句首表示目的，以修饰之后动词原形start起首的命令句。原句实等于：
If you want to prepare a meal for about 50 guests, (you should) start by calculating how much food you want to prepare per guest.
同理：
[例] To succeed, you should work hard.
=If you want to succeed, you should work hard.
(你想要成功，就应该努力。)
根据上述，(C)应为正选。