Read each sentence to find out whether there is any grammatical mistake/error in it. The error if any will be in one part of the sentence. Mark the number of the part with error as your answer. If there is 'No error' mark(d).
`("I do not know")/a` `("what is he doing?")/b` `("to solve the problem")/c` `("No error")/d`.

Explanation

(b) Use .what he is doing.