Are the underlined words in the sentence an independent clause, a subordinate clause, or neither?

is that the man whom we read about yesterday?

a.
neither

b.
subordinate clause

c.
independent clause

Since there are no underlined words in the sentence, let me answer this way:

The independent clause is "Is that the man" while the subordinate clause is "whom we read about yesterday". An independent clause can stand alone as it expresses a complete thought while a subordinate clause needs the independent clause to function properly.