One of the greatest hurdles facing middle school students learning English is relative pronouns

The problem quoted isn't one, but there are problems in the reading section that ask you to distinguish relative pronouns from relative adverbs

However, the preposition+relative pronoun (which) part becomes a relative adverb (where)

Relative pronouns perform as 'conjunction + pronoun'

The relative pronoun 'that' has two states, a nominative case and objective case, but there is no possessive case