The correct option is C 10,000
The unique code has 4 digits.
Each digit is formed using any of the 10 numbers from 0 to 9.
Hence, a single-digit number can be formed in 10 ways.
A double-digit number can be formed in 10 x 10 ways.
A three-digit number can be formed in 10 x 10 x 10 ways.
A four-digit number can be formed in 10 x 10 x 10 x 10 = 10,000 ways.