Until the bar line

An accidental does not apply only to its own note: it applies to every note in that same position until the bar line. If an F♯ appears at the start of the bar, any other F on that same line or space, later in the bar, is also sharp even without a sign.

The bar line wipes it out: in the next bar, F is natural again.

To cancel an accidental inside the same bar, use the natural sign (♮). The same rules apply: until the bar line.

• Same bar, same position, no sign: the accidental still holds.
• New bar: everything is natural again.
• Natural sign: cancels the accidental from that note on.

In the exercise one note is marked and you say how it sounds. Check for an accidental earlier in the bar.