What indicates that a singular entity possesses something?

The correct choice highlights the use of an apostrophe before the letter "s" as a grammatical tool to indicate possession for singular nouns. For instance, in the phrase "the dog's leash," the apostrophe combined with the "s" signifies that the leash belongs to the dog, thereby establishing a clear relationship of ownership. This form is essential in English grammar, helping to convey that a singular entity possesses something distinctly.

Other choices, while important in their own contexts, do not serve the function of indicating possession. For instance, commas are utilized for separating elements in a sentence, quotation marks are used to denote spoken dialogue or direct quotes, and semicolons serve to connect independent clauses, none of which express ownership or possession directly. Thus, the apostrophe before "s" stands out as the correct indicator of singular possession.