Concept:The sentence requires the simple past passive voice because an action was done to the subject in the past.
Explanation:The original underlined phrase "was restrict as an" incorrectly uses the base form of the verb "restrict".
The sentence describes a past event: his entry was prevented at that time.
In the simple past passive, the structure is: Subject + was/were + past participle of the verb.
Here, the subject "entry" is singular, so we use "was". The past participle of "restrict" is "restricted".
Thus, the correct phrase is "was restricted as an".
Option A ("is restricting as an") is present continuous, which does not match the past tense context.
Option B ("was restricting as a") uses the past continuous active, not passive, and the article "a" is incorrect before "official" (which starts with a vowel sound).
Option D ("is restricted as a") is simple present passive, not past, and again uses wrong article "a".
Option E ("No improvement") is incorrect because the original has a grammatical error.
Therefore, option C ("Was restricted as an") is the correct improvement.
Answer:C. Was restricted as an