Unable to defrag HDD partition with OS on it

When I click on optimize drive OS (C:), it begins by analyzing and then goes onto to step1 where it says it is relocating.

In 30 seconds, it states that the drive is OK(10% fragmented).

How does one manually defrag this drive?

I have an Inspiron 15 3521 that came with Win 8 nd I have upgraded to 8.1 via Win Store.

Thank you.