The pronoun 'who' can be singular or plural.
Examples:
Who is that man I saw you with?
Who are those people picketing our shop?
The possessive form of the pronoun 'who' is whose, also singular or plural.
Examples:
Whose car is blocking the driveway?
Whose cars are blocking the street?
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.
Quantum is singular, not singular possessive. The singular possessive form is quantum's.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive pronouns are mine, yours, his, hers, and its. The singular possessive adjectives are my, your, his, her, and its.
The singular possessive of dish is dish's
Grave's is the singular possessive.
The singular possessive is test's.
The singular possessive is Martin's.
The singular possessive is fish's.
Buzz's is the singular possessive.
The singular possessive is biker's; the plural possessive is bikers'.
The singular possessive of criteria is criteria's.
There's no such thing as a singular possessive verb. Chris's is a singular possessive noun.