When should an apostrophe be used to pluralize letters or numbers?

The correct choice addresses the use of apostrophes when pluralizing letters and numbers. Specifically, an apostrophe is used to clarify the meaning and ensure readability when plural forms of letters or numbers may be confusing. For instance, if you were to pluralize the letter "A," it would be expressed as "A's" to indicate that it's referring to multiple instances of that letter. Similarly, when dealing with numbers, you might say "There are two 7's in that calculation," to indicate more than one "7."

In contexts where the pluralization may lead to misunderstandings without the apostrophe, using it helps provide clarity. This convention is particularly useful in educational or written settings where precise communication is essential.