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 singular possessive form of "test" is "test's".
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 is ant's.The plural possessive is ants'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive of dish is dish's
Taco's is the singular possessive.
The singular possessive form is classmate's.
The singular possessive is biker's; the plural possessive is bikers'.
The singular possessive is fish's.
Buzz's is the singular possessive.
Grave's is the singular possessive.
The singular possessive is Martin's.