Explanation:

- "On tenterhooks" is an idiom originating from the textile industry, where cloth was stretched out to dry. It means being in a state of suspense or anxiety.

- Option 1: unhappy - While being on tenterhooks might make one uncomfortable, it specifically relates more to suspense than to sadness.

- Option 2: angry - Anger is not related to the meaning of "on tenterhooks," as the idiom conveys anticipation or worry rather than irritation.

- Option 3: anxious - This correctly captures the essence of the idiom as it describes a feeling of nervousness and anticipation during moments of uncertainty.

- Option 4: happy - This does not align with the idiom, as being on tenterhooks implies a lack of comfort and certainty.