The use of "has completed" is in the present perfect tense, which is used for actions that happened at an unspecified time before the present. However, the sentence refers to a future event, and the action should be framed in the future perfect tense instead. In this context, the correct verb form is "will have completed" to match the future timeline being discussed.
Corrected version: "By the time he has completed his work, we will have already started the meeting." should be changed to "By the time he will have completed his work, we will have already started the meeting."
Hence, option (a) is the answer.
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