Which of the following statements is not true?

a. "whom" is an object pronoun.
b. "who" is a subject pronoun.
c. "whom" can serve as the object of a preposition.
d. "who" and "whom" are personal pronouns.

"Whom" can serve as the object of a preposition.
True; Ex. With whom did you go to the mall?

"Whom" is an object pronoun.
True; Ex. To whom did you give the book?

"Who" and "whom" are personal pronouns.
False; Who and whom are relative pronouns. Personal pronouns are: I, you, he, she, him, her, it, we, they, us, them. 

"Who" is a subject pronoun.
True; Ex. Who is running?

The main idea exclaims how everyone has mercy, even the greatest of kings yet have mercy.