Final answer:
The possessive form of a singular noun, including those that end in -s, is formed by adding an apostrophe followed by an 's', making 'cat's' the correct form to indicate possession.
Step-by-step explanation:
The possessive form of a singular noun, even a noun that ends in -s, is typically formed by adding an apostrophe followed by an "s" ('s).
For example, if you are indicating possession for a singular noun like "cat," you would write cat's, showing that something belongs to the cat. This rule applies regardless of whether the noun ends with the letter -s or not.
Therefore, the correct possessive form for a singular noun is option a) cat's.
Common Apostrophe Errors to note include not using apostrophes to form plural nouns or to indicate possessive pronouns.