If you haven't clicked "Yes" DO NOT. You will create a dynamic disk which will be a headache for you.
Windows 7 can only have a maximum of 4 primary partition. Many computer suppliers configure their systems with 4 primary partitions like yours. For more partitions you will need to make the data disk D: a logical partition. Then you can make more logical partitions without your disk turning into a dynamic disk.
Here is a tutorial Partition or Volume - Create New
Before you make any changes to your disk partitioning make sure you have backed up. It can always go wrong.