Sahil said to Dilu, "I was going to buy milk".

A Sahil told Dilu that he were going to buy milk.

B Sahil told Dilu that he would going to buy milk.

C Sahil told Dilu that he had been going to buy milk.

D Sahil told Dilu that he was going to buy milk.

Correct Answer: Option C

When Reporting verb in past Tense and reported speech is having "was" after subject + First Verb.

Then we change "was" to "had been" + First verb.