[susan53]

First of all, remember what I've said again and again. It's not a matter of rules but of meaning/context. Trying to combine the propositions (ideas) without knowing the context makes no sense. So for instance

a) If you mean The hotel has a very big swimming pool , then you could also say: The hotel has a swimming pool which is very big Here which (or that) refers to the swimming pool and defines a specific quality of the pool.

b) But look at the following:
A. There's a small hotel and a big one. They're both nice, but we prefer the big one.
B : Why?
A: Well, the small hotel is family run and very friendly, but the other one - the hotel that's bigger - has a lovely swimming pool.
This time, that (or which) refers to the hotel. Again it's defining and explains which of the two hotels I'm talking about.

So in your example : The hotel has a swimming pool. It is very big. the lack of context means that it's impossible to say what "it" refers to. (The hotel? Or the swimming pool?) And this is why you can't decide how to combine them. It depends entirely on what you want to say. The writer of the activity was obviously working on the assumption that, without a context, "it" should be understood as referring to the last singular noun - but if you add a context it wouldn't necessarily be true.

Whether it's defining or not has no connection with the article. Again it's a matter of meaning. Defining clauses explain (ie define) what/who exactly you're talking about. Non-defining clauses simply give extra information. For example, in what follows who lives in America defines which brother i'm now talking about:

A : Your brother David lives in America, doesn't he? And Anne told me he's a doctor.
B: No actually I have two brothers. My brother David lives in Scotland and he's a lawyer. My brother who lives in America is Steven. And yes, he's a doctor.

And compare it with:
She has a brother and a sister. The brother, who lives in Scotland, is called David and he's a lawyer.
This time it's not necessary to "define which brother" I'm talking about, because there's only one. Notice the commas, (which could also be parentheses or dashes, and would become a slight pause if spoken). They show that the clause is non-defining and just contains extra information about the referent. The relative clause could be taken out of the sentence and it would remain totally comprehensible.

Notice incidentally that that can only be used in defining relative clauses (as an alternative to which/who). In non-defining clauses that is impossible and which/who obligatory.