Question

5. Einstein had proposed the concept of photon to explain the photoelectric effect.(change into passive voice)​

Answer 1

Answer:

to explain the photoelectric effect,Einstein had proposed the concept of photon

I think it's a correct answer

Answer 2

Answer:

The concept of photon to explain the photoelectric effect had been proposed by Einstein.

Explanation:

The simple past (had) is changed to past perfect tense (had been)