The pronoun 'who' functions as both singular and plural, as an interrogative pronoun and as a relative pronoun. Examples:
Interrogative pronoun
Who is your new math teacher?
Who are your visitors?
Relative pronoun
Mr. Madden, who teaches math and chemistry, is my new teacher.
The visitors, who are my cousins, are staying for the weekend.
The singular possessive form of "test" is "test's".
The singular possessive pronouns are "my," "mine," "your," "yours," "his," "her," and "its."
The singular possessive of "ant" is "ant's" and the plural possessive is "ants'".
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form of "classmate" is "classmate's."
The singular possessive of dish is dish's
Taco's is the singular possessive.
The singular possessive is biker's; the plural possessive is bikers'.
The singular possessive is Martin's.
The singular possessive is fish's.
Buzz's is the singular possessive.
Grave's is the singular possessive.