Which of the following is the smallest storage unit?

The correct order of storage units from smallest to largest is:

Kilobyte (KB)

Megabyte (MB)

Terabyte (TB)

Petabyte (PB)

“A kilobyte is approximately 1,000 bytes. It is smaller than a megabyte (1 million bytes), terabyte (1 trillion bytes), or petabyte (1 quadrillion bytes).” — CompTIA ITF+ Reference

Hence, C. Kilobyte is the smallest unit listed.