Degrees of Adjectives conversion for "not one of the largest"..
Considering the other example given by OP:
Superlative: Wheat is not the most nutritious of all the grains.
Positive: Some grains are at least as nutritious as wheat.
The Comparative form as per rule would be :
Wheat is not more nutritious than some other grains.
Correct answer.
Now applying this to OP's question:
Question :- Usha Kiran is not the tallest of all the buildings in the city.
The correct answer should be:
Usha Kiran is not taller than some other buildings in the city.
OR.
Some other buildings in the city are taller than Usha Kiran
Discussion on options given in the question.
1.Some other buildings in the city are at least taller than Usha Kiran.
2.Some other buildings in the city are perhaps taller than Usha Kiran..
Option 2 is incorrect because the word "perhaps" is used. In the question it is mentioned Usha Kiran is not the tallest building, hence there are undoubtedly other taller building/ buildings in the city. The second option would work if we remove "perhaps".
Option 1 combines positive and comparative transformations of the given sentence, but it is invalid. When comparing two structures, one can be "at least as tall as" but not "at least taller than." This is an invalid sentence structure. The questioner had intended to invent a new phrase and may accept this as the correct response. But it doesn't work.
To summarise: Neither of the options is correct.