Did he have to help his sister the bed yesterday

选a正确,考help sb (to)do sth,也就是说help 的宾补不定式不带to 。但必须记住感官动词、使役动词的宾补不能带to,象feel have make 等。但这些动词的宾补要是变成主补就要带上to 了。例如, The landlord made the farmhand work long hours a day.变为被动就是The farmhand was made to work long hours a day .此外,还要注意help 的其它重要用法。比如,be of great help相当于be helpful,还有它做名词的用法也必须记住,比如, The dictionary is a great help for me to study a language.

a
help sb (to) do sth
to可省略