Step-by-step Solution:

The correct answer is: B. Gigabyte

Explanation:
1 Gigabyte (GB) = 1,000 Megabytes (MB) or 1 billion bytes.
Here's the hierarchy of memory units:
Kilobyte (KB) = 1,000 bytes
Megabyte (MB) = 1,000 Kilobytes (1 million bytes)
Gigabyte (GB) = 1,000 Megabytes (1 billion bytes)
Terabyte (TB) = 1,000 Gigabytes (1 trillion bytes).