To solve this question, the user needs to know the correct command to delete all files with a certain file extension.

Now, let's go through each option and explain why it is right or wrong:

A. Del .txt: This command will not work as it deletes only files with the exact name ".txt", not all files with the ".txt" extension.

B. Erase .txt: This command will not work as it deletes only files with the exact name ".txt", not all files with the ".txt" extension.

C. Del *.txt: This command is correct. It will delete all files with the ".txt" extension.

D. Del ./p: This command is incorrect. The "/p" option is not a valid option for the "del" command, and the command does not specify which file extension to delete.

Therefore, the correct answer is:

The Answer is: C