T'ai Chi is my favorite and I often play it _____ healthy.
本题考查动词不定式作目的状语的用法。句意：太极拳是我最喜欢的（运动），为了保持健康我经常打太极拳。to keep healthy作目的状语。
I'm going to but a birthday present for you. You will receive it ______ post.
本题考查by post的固定搭配。该短语意为“以邮寄的方式；通过邮寄”，故答案为B。
—How long does it take you to finish your homework every day?
—I usually ______ two hours on it.
答语的含义是“我通常花两个小时在家庭作业上”，根据主语 I 以及其后的 on it 可知应用 spend。故答案为A。