here is the explaination I got after answering the quesiton incorrectly...
'SQL Server 2005 does not enforce the .mdf, .ndf, and .ldf file name extensions, but these extensions help you identify the different kinds of files and their use. MDF is for primary database files, NDF is for secondary database files and LDF is for log files.'
I left out the log file, but in thinking about it - I do agree that the log file is part of the database files. I am not hung up on whether I get or don't get the point, but I find it funny that the explaination supports my incorrect answer.