Era has been ill ________ three months.

The correct answer is "for."

Explanation:

- Since: This preposition is used to indicate the starting point of an action or situation. However, "since" is typically followed by a specific point in time, not a duration. For example, "since January" or "since last year."

- About: This preposition is used to indicate a general time frame or approximation. However, it doesn't convey the duration of time as accurately as "for" does.

- In: This preposition is used to indicate a specific point in time or a period within which something occurs. However, it doesn't convey the duration of time as clearly as "for" does.

- For: This preposition is used to indicate the duration of time during which something has happened or will happen. In this sentence, "for" correctly conveys the duration of three months during which Era has been ill.