who or whom

sinbadfromtheboardWhat Why does the answer key say "who" is the correct pronoun ?

Because it is. 'Who' is the subject of 'would be'.

An easy way to remember (at least for me) is that 'who' applies to 'he' and 'whom' applies to 'him'.

Who would be the best performer?
He would be the best performer.

I write this letter to whom it may concern.
This letter concerns him.

The standard way to test out who/whom dilemma is to replace the pronoun with either he/she or him/her.

Since this sentence has two verb phrases, how does one determine where to place the he/her/him/her within the sentence.

In this instance, does "The human resource director hired an individual" act as a participle phrase ? If so, why ?

But why can you not say, "The human resource director hired him" ? Him is objective case, therefore whom is the correct relative pronoun.

sinbadfromtheboardThe human resources director hired an individual (who/whom) he though would be the best performer.

You divide it up like this:

1 The human resources director hired an individual
2 who .......................would be the best performer.
3 ........( he thought )

All three parts have a subject: director, who, he

How can part 3 be divided without a direct object ? Isn't thought a transitive verb ?

Basically, how do you determine where to divide parts 2 and 3, as you have them listed ?

In The director thought, "He will be the best performer", you can, if you wish, consider the clause in red to be the object of thought. Within that clause, He is the subject.

"The human resources director hired an individual who (he though) would be the best performer."

"The human resources director hired an individual who (he thought) would be the best performer."

If you remove 'he thought', it should be clear that 'who' is the correct answer.