Answer:
C. Sloane and Herring's law firm will represent the client.
Step-by-step explanation:
A singular possessive noun is used to denote ownership and is done by adding an apostrophe (') to the noun, but however, when it is more than one noun, the ('s) is used and it becomes a plural possessive.
Therefore, the sentence that contains an incorrect usage of the singular possessive is option C, because it is a plural possessive noun.