Answer: a) that.
Explanation:
A relative clause is a clause that starts with a relative pronoun, and provides more information about the noun that precedes the clause. The most common relative pronouns are who, whom, whose, which, and that ( but where is also sometimes used as a relative pronoun).
In the sentence 'Last Friday was the day that Karl won the marathon', there is a noun - day, followed by a relative clause 'that Karl won the marathon', which gives us more details about that particular day.
