is it possible to leave out the relative pronoun "where"? [duplicate]

Look at this question:

The Think Tank is the only place in the world _____ visitors have free access to this type of research.

a. who b. that c. which d. Ǿ e. where f. when

The answer says that both d. and e. are correct. Do you agree with it? If d is correct, why are b and c incorrect?

Solution 1:

The Think Tank is the only place in the world _____ visitors have free access to this type of research.

Relative clauses which occur after noun phrases headed by the word place behave in an exceptional way. Relative clauses have an antecedent, which is the phrase that is being modified by the relative clause, the thing that we are giving more information about. Some nouns, in particular the word place, can have relative clauses where the gap in the relative clause represents a locative phrase of the type that would normally be represented by a preposition phrase within the relative clause. This is not normally possible unless the relative clause uses the relative word where or that. However, when place - or a small number of similar words - is the antecedent noun, it is possible without any relative pronoun. Consider the following sentences which are grammatical when the antecedent is the word place, but not if it is another noun:

That's the place we ate last time
Dublin is the place I want to live.
That's the place we saw that incredible film.
This is the place they kept the prisoner.

Compare those with the following:

*That's the restaurant we ate last time. (ungrammatical)
*Dublin is the city I want to live. (ungrammatical)
*That's the theatre we saw that incredible film. (ungrammatical)
*This is the dungeon they kept the prisoner. (ungrammatical)

And then consider these, which do use the relative preposition where:

That's the restaurant where we ate last time.
Dublin is the city where I want to live.
That's the theatre where we saw that incredible film.
This is the dungeon where they kept the prisoner.

The Original Poster's example

The antecedent for the relative clause in the Original Poster's example is the noun phrase:

the only place in the world

Because this phrase is headed by the word place options (b), (d) and (e) are acceptable. If the antecedent didn't contain the word place (e) would not be so viable. For example, if we replace the place in the world with the word library we get the following - which is completely ungrammatical: