In the following question, a sentence is given with a part in bold. The given phrase in bold may contain an error. The options following can replace the incorrect phrase. The correct phrase that is to be replaced will be your answer. If the sentence is correct as it is, then select ‘No improvement required’ as the answer.

Despite numerous attempts, the problem remained so unsolvable as it was before.

The correct option is as unsolvable as it was before.

Step-by-Step Explanation:

1. Understanding Comparative Structure:
When making an equal comparison between two states or conditions, the correct correlative conjunction structure is "as + adjective + as".

2. Analyzing the Original Sentence:
The phrase in the given sentence, "so unsolvable as it was before", uses an incorrect comparative pair ("so ... as"). While "so ... as" can sometimes appear in negative comparisons, the standard positive structure for expressing equality is "as ... as".

3. Evaluating the Options:
- as unsolvable as it was before: Correct. It properly uses the "as ... as" construction to indicate equality, and the singular pronoun/verb "it was" correctly agrees with the singular noun "problem".
- as unsolvable than it was before: Incorrect. "Than" is used with comparative adjectives ending in "-er" or preceded by "more", not with "as".
- as unsolvable as it were before: Incorrect. Subject-verb agreement is violated because "problem" is singular, requiring "was" instead of the plural/subjunctive "were" in this straightforward past-tense comparison.
- as unsolvable that it was before: Incorrect. "As ... that" is ungrammatical for expressing direct comparison.

Therefore, replacing "so unsolvable as it was before" with "as unsolvable as it was before" makes the sentence grammatically correct and coherent.