Explanation:

- Being occupied with(A)/work: This part means someone's time is taken up by work. The grammar is correct.

- Father had no(B)/time to see us(C): This part indicates that the father was too busy to see his children. There's nothing wrong here grammatically.

- No error(D): This option suggests that there's no grammatical error in the sentence.

- Option:1, a: Indicates an error in part A, which isn't the case here.

- Option:2, b: Indicates an error in part B, which isn't the case here.

- Option:3, c: Indicates an error in part C, which isn't the case here.

- Option:4, d: Implies there is no error in the entire sentence.

The correct option is Option:4, d.