Step 1: Reverse tense shift.
In indirect speech, “was not working” is past continuous, but in direct speech it can correspond to present continuous (“are not working”) if the reporting verb is in past tense but refers to a current state.
Step 2: Maintain subject-object relationship.
Pronoun “he” in indirect speech becomes “you” in direct speech, as the speaker is addressing him directly.
Step 3: Choose correct reporting verb structure.
Option (a) correctly uses “said to him” as the reporting verb for direct speech. Option (b) is grammatically incorrect (“told to him” is wrong), and option (c) omits the object “him,” changing the meaning.
\[
\boxed{\text{(a) I said to him, “You are not working hard.”}}
\]