Chose the correct relative pronouns in accordance with the sentence

I know the candidate ___ I am going to support in the next elections.

The correct answer is: Whom

The correct answer is "Whom".
Here, we get two distinct sentence segments, namely "I know the candidate", and "I am going to support the candidate in the next elections". "The candidate" is the common portion which must be substituted for a relative pronoun to link the two parts. Among the given options, "Whom" is the most suitable alternative to suggest the receiver of an action(supporting in elections). So, the correct answer is "Whom".