The correct answer is option 'D'.

Explanation:
The relative pronoun "which" is used to refer to things or animals. In this sentence, the van is a thing, so "which" is the appropriate pronoun to use.

Relative pronouns:
Relative pronouns are used to connect a clause or phrase to a noun or pronoun. They introduce relative clauses, which provide additional information about the noun or pronoun.

The relative pronouns in English are: who, whom, whose, which, and that.

Examples:
- The woman who lives next door is a doctor. (refers to a person)
- The book that I borrowed from the library is very interesting. (refers to a thing)
- The dog whose tail is wagging is very friendly. (refers to a possessive relationship)
- The car which was parked outside my house was stolen. (refers to a thing)
- The person whom I saw at the store was my neighbor. (refers to a person)

Usage of "which" in the given sentence:
In the given sentence, the van is the noun being referred to. The relative pronoun "which" is used to introduce additional information about the van.

The sentence is structured as follows:
- The van, (noun being referred to)
- which (relative pronoun)
- was stopped earlier today by police, (additional information about the van)
- turned out to be driven by the men suspected of last week's bank robbery. (additional information about the van)

Therefore, the correct relative pronoun to use in this sentence is "which."

Incorrect options:
a) "Who" is used to refer to people, not things. It is not appropriate to use in this sentence.
b) "Whose" is used to show possession, not to introduce additional information about a thing. It is not appropriate to use in this sentence.
c) "Whom" is used as the object of a verb or preposition, not to introduce additional information about a thing. It is not appropriate to use in this sentence.