He ( ) read and write at the age of 6 .

could
他六岁时就会看书写字了.
再问： 没这个选项，选项是A must B might C would D was able to
再答： D 这题目没出好。 这里只能选D 了。 was able to 一般指“具体某一次“过去能够做成某事”， could 泛指过去某方面的能力， 显然could 好些。