Question

He - do this work if he wanted
a) could
b) might
c)would
d) should
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Answer 1

Only the sentence “He could do it if he wanted to” is correct, because could is related to the past (could is the past of can).

In the second sentence, as wants refers to the present, so could have to be referred as well: “He can do it if he wants to.”

Answer 2

He could do this if he wanted