The first option is the most expensive option that you will travel to an overseas country.

 (Grammar Questions I Couldn't Answer)

This is one that (with the help of some colleagues in the staffroom), I think I did actually find an answer for eventually.  But I'd appreciate a second opinion if anyone else wants to have a look at this.  Did I get the right answer in the end?

When marking a student’s speaking video, I came across the following sentence: "The first option is the most expensive option that you will travel to an overseas country."

My native speaker intuition told me that something was wrong, and having taught relative clauses recently, I was pretty sure I knew what it was.  The relative pronoun “that” was referring back to “option”, but “option” was neither the subject or the object of the following clause, so the relative clause was incorrectly formed.
My native speaker intuition was also telling me that the sentence could be fixed by replacing "that" with "which is that"--i.e.   "The first option is the most expensive option which is that you will travel to an overseas country."  But I couldn't figure out what the explanation for this was so I couldn't explain it to the student.

I asked some colleagues around the staffroom, and we discussed and batted some ideas around, and looked in some grammar books.  Eventually, we decided that "...which is that you will travel to an overseas country" is not actually a relative clause, but what the grammar books called a "that-clause".  

So I ended up writing the following feedback to the student:

"The first option is the most expensive option that you will travel to an overseas country."--This is a good attempt at a relative clause, but you can't actually use "that" as a relative pronoun here, because (as we discussed in class) the relative pronoun has to be the subject or the object of the next clause.  As a relative pronoun, "that" would refer back to the noun preceding it (in this case "option").  So you could say, for example:  "This is the most expensive option that you can choose"---(the relative clause is "You choose that" mean "You choose the option").  But you can't say "that you will travel to an overseas country."  You could, however, use "which" to refer to the previous clause (see p.76 of the textbook--"which" can refer to the whole of the previous clause").  You could then connect which to a "that clause"--(which is a declarative clause, different than a relative clause) and say "The first option is the most expensive option, which is that you will travel to an overseas country."

What do you think?  Did I explain this accurately to the student?