In MS-DOS you can use small or capital letter of combination of both to enter a command but internally MS-DOS work with.

Answer: Option B

In MS-DOS, commands can be entered using either small or capital letters, or a combination of both. However, internally, MS-DOS processes commands in capital letters.
Option A: Small letter - Incorrect. MS-DOS does not process commands in small letters internally.
Option B: Capital letter - This is the correct answer as MS-DOS converts all commands to capital letters for internal processing.
Option C: Both a and b - Incorrect. MS-DOS does not use both small and capital letters internally; it uses only capital letters.
Option D: None of above - Incorrect. The correct option is B, as MS-DOS uses capital letters internally.