Change the following sentence to passive voice.Nobody can solve this problem.

The sentence changes into passive voice use of the form this. 

Simple present form, passive voice takes the simple past form.

Subject + verb + object ( active voice ).

Therefore a, b, d all options are wrong according to the form of passive voice.

So, the option C is correct.

( C ) This problem could not be solved by anyone.

Final Answer:-  ( C )This problem cannot be solved by anyone.