Some students are confused with the options A and C.
She told me that her father _____ to the post office when I arrived.
Could you tell me whether C is the only correct answer in this question?
A, C, and D are correct, but C is most likely the intended answer.
She told me that her father just went to the post office when I arrived.
This means that when you arrived, the father went to the post office, and did nothing else beforehand. The father did not leave before you arrived, and wasn't even in the process of leaving. He might have left as a result of your arrival. This is grammatical, but the "just" is a little odd, because it implies we should expect the father to do something else as well, which he did not do.
B is incorrect, you can't use the present tense to describe an event that has already happened.
C is probably the intended interpretation. Shortly before you arrived, the father left, and was already gone when you arrived.
D is also fine, it says that when you arrived, the father was already in the process of leaving, maybe he'd just started the car to drive away.
According to grammar, the speech in direct speech is as follows:
She said to me, "My father just went/just had gone to the post office when you arrived".
The sentence in reported speech should be as follows:
She told me that her father had just gone to the post office when I had arrived.
