To answer this question, the user needs to know what is meant by a filter command.

A filter command is a command that takes input from standard input, performs some operation on it, and then produces output to standard output.

Now, let's go through each option and explain whether it is a filter command or not:

A. man: This command is not a filter command. It is used to display the manual page of a given command.

B. pg: This command is a filter command. It is used to paginate the input that is received from standard input.

C. cat: This command is a filter command. It is used to concatenate files and display the output on standard output.

D. head: This command is a filter command. It is used to display the first few lines of the input that is received from standard input.

Therefore, the answer is:

The Answer is: A. man