Rahim found a laptop in the classroom, but could not determine ____ it belonged to.
Which is correct for the above gap
Who/whose/who's/ who is
Please give an Explanation also.
I highly recommend the famous book English Grammar in Use by Murphy. It has explanations, exercises, and answers to the exercises. Consult Units 49-50 and 92-96.
Rahmi found a laptop. To whom did it belong? [archaic]
Rahim found a laptop. Who did it belong to? [who as object of preposition "to"]
Rahim found a laptop, but [he] could not determine who it belonged to.