Hence, option D is the correct answer.

The sentence is grammatically incorrect, and the error lies in option D.

“So” is placed before a modifier — adjective or adverb — to intensify its quality or manner.

This emphasis on the modifier expresses a remarkable situation.

This clause can stand alone as a sentence, or it can be paired with that + clause to express an effect (result) of the remarkable situation mentioned in the main clause.

For Examples: “He was so drunk that he could not walk straight”.

Thus, “as” must be replaced with “so” to make the sentence grammatically sound. The correct sentence will be: Our soldiers fought so fiercely that the enemy had to accept defeat.

Option B has the grammatically incorrect part. The sentence talks about one single cab and the verb used before it is ‘were’ and hence is erroneous. So, we will use a singular verb instead of plural; thus, ‘was’ will be used in place of ‘were’.

Option A has the grammatically incorrect part. The use of the phrasal verb ‘meted on’ is wrong and ‘meted out’ should be used in its place. ‘Meted out’ is usually used to refer to something unpleasant (such as punishment).

e.g. Huge fines were meted out as punishment.

Option A has the grammatically incorrect part. The use of ‘fastly’ is wrong in the given sentence and ‘fast’ should be used in its place. The word ‘fastly’ is not the correct adverb to qualify the verb ‘walk’. The correct adverb is ‘fast’ which should be used in the given sentence.

Option D has the grammatically incorrect part. The use of ‘ones’ in the given sentence is wrong. The subject of the sentence ‘the current situation’ is singular, so ‘one’ should be used in place of ‘ones’.

Option C has the grammatically incorrect part. The first phrase of the sentence starts with the phrase “no sooner” which is followed by ‘than’ not ‘when’. ‘When’ is used with ‘hardly’ or ‘scarcely’.

Hence, option A is the correct answer.