Final answer:
In the question, the 'whom' is used as relative pronoun since it refers to the guests, the object of the preposition 'of'. Also, 'who' can be used as a relative pronoun in cases where it refers to the subject of a clause.
Step-by-step explanation:
The question asks to join two sentences using a relative pronoun. In English grammar, we use 'who' and 'whom' as relative pronouns to refer to people. 'Who' is a subjective case pronoun, and 'whom' is an objective case pronoun.
The sentence combination would go as: "The guests whom I was speaking of have arrived." The word 'whom' correctly fits into this sentence because it is referring to the objects 'guests' which I was speaking of.
You can also use the sentence structure: "The guests, who I was speaking of, have arrived." Here, 'who' can be used as it refers back to the 'guests' - the subject of the clause 'I was speaking of'.
Learn more about Relative Pronouns