Correct Answer: (d) When Mrs. Sinha came to see Varanasi in 2019, Nupur has already been teaching there for five years.
Solution:'Has been' must be replaced 'had been' as the first clause of the sentence is in Past Tense. So, we need to use "Past Perfect Tense (Had been + V-ing)" to make the sentence grammatically correct. Hence, 'Nupur had already been teaching(V₁- ing)' is the most appropriate answer.