Explanation:

Correct Answer is (d).If an adjective is to be modified, an adverb can only be used. In this sentence, ‘low’ is an adjective and ‘alarming’ is also an adjective’. Therefore the sentence is not grammatically correct. An adverb must precede the adjective ‘low’ in this statement. Hence, it should have been ‘alarmingly’.
The correct sentence will therefore be:
After the medical examination, he got to know that his blood sugar level was alarmingly low and he had to be hospitalized immediately.”Clearly, Option D is the correct choice.