A portion of the sentence below is underlined. Select the most suitable replacement to make the sentence correct. If it is already correct, choose ‘No improvement required’.

The gallery closed unexpectedly, so I had to return the following morning to see the exhibition.

The correct option is No improvement required.

Step-by-step Explanation:

1. Analyze the original sentence structure:
The original sentence is: "The gallery closed unexpectedly, so I had to return the following morning to see the exhibition."
- The initial clause "The gallery closed unexpectedly" establishes a past event using the simple past tense (closed).
- The main clause expresses a past necessity using the past modal phrase had to followed by the base form of the verb return (had to return). This maintains tense consistency across the sentence.
- The phrase "the following morning" is the appropriate time expression used in past narratives.

2. Evaluate the given options:
- No improvement required: The original sentence is completely correct in grammar, tense agreement, and logical flow.
- "so I had returned the following morning": Using the past perfect tense (had returned) is incorrect because it implies that returning happened prior to the gallery closing.
- "so I have to return the following morning": The present tense (have to) creates an inconsistent shift in tense from the past event (closed).
- "so I had to returned the following morning": This contains a grammatical error; the phrase had to must always be followed by the base form of the verb (return), not a past-tense form (returned).

Conclusion:
Because the original sentence correctly applies tense consistency and grammatical structure, no changes are needed.