Concept:The sentence is grammatically correct with proper verb forms and adjective usage.
Explanation:The verb "requires" is in the present indefinite tense, which is appropriate for a general fact.
After "requires", the structure "one to fast" uses the infinitive form correctly.
The phrase "after which" connects the clauses properly.
The word "fasting" here functions as an adjective describing "blood sugar level", which is correct.
Thus, all underlined parts (a), (b), and (c) are error-free.
Answer:(d) No error