Mitch help

aflaam wrote:My doubt is as we know no two pronouns can have same referent.
But in D, which and their have same referent.
What am i missing?

It sounds as if possibly you have misunderstood something or have been misled.

I have not really considered the rule that you are referring to, but let's try a simple hack, creating sentences that are clearly correct and have multiple pronouns referring to the same referent.

As John ran out the door, he lost his keys.

Writing As John ran out the door, he lost John's keys. would be absurd. So clearly using a personal pronoun and then a possessive pronoun both of which refer to the same person is OK.

The car that is going down the street is missing one of its hubcaps.

Writing The car that is going down the street is missing one of the car's hubcaps. does not make sense either.

Here's another one.

Andrew was finally able to locate the woman who had interviewed him when he had applied for his previous job.

In the that sentence, him, he and his all correctly refer to Andrew.

So the rule you have cited is different from how you understood it to be.

I think that what you are talking about may have something to do with situations involving relative pronouns.

For an example, look at the following sentence.

The dog that ate the food, which ran across the street, is still around somewhere.

I am not sure that there is a rule that says that that and which cannot refer to the same noun, but that sentence is a bit awkward, and it also seems to convey that the food ran across the street.

In any case, maybe you should go back to the source that you got this idea from and get a clearer sense of what was said about pronouns.

aflaam wrote:But in D, which and their have same referent.

Unlike the short flights of the shuttle and earlier spacecraft, which were capable of carrying sufficient power in fuel cells and batteries for their short flights, a permanently orbiting space station will have to generate its own electricity.
Here, which serves to refer to flights, while their serves to refer to which.
Grammatically, their does not directly refer to flights.
Rather, their serves to refer to the preceding plural subject (which).