Final answer:
The word "which" in the sentence is a relative pronoun, used to introduce a relative clause and give more information about the noun "small companies."
Step-by-step explanation:
The word in question from the sentence "Small companies, which could reach only local customers in the past, now sell to a worldwide market." is a relative pronoun. Relative pronouns are used to connect a clause or phrase to a noun or pronoun. The common relative pronouns include "who," "whom," "whose," "which," and "that." In this sentence, the relative pronoun "which" is used to introduce the relative clause "which could reach only local customers in the past" and gives more information about the noun "small companies." Hence, the correct answer to the question is "a) Relative Pronoun."