Question

form a compound sentence.
_____________________________ can fit in your pocket now.
a computer once filled an entire room but
modern computing devices, such as cell phones,
a computer once filled an entire room, but one

Explanation:

To form a compound sentence, we need two independent clauses joined by a conjunction (here "but"). The first option lacks a complete second clause. The second option is a phrase, not a clause. The third option has "A computer once filled an entire room" (independent clause) and "one can fit in your pocket now" (independent clause) joined by "but", making a proper compound sentence.

Answer:

C. A computer once filled an entire room, but one