Is your C drive a Solid State Drive (SSD)?
If it is then you should use the 'Optimize' option of defraggler which is designed for SSDs, rather than 'Defrag' which is designed for HDDs.
From the support article on the different types of defragmenting and optimizing:
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<strong>Optimize:</strong>
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This is designed for Solid State Drives, aka SSDs. Specifically, it will initiate the TRIM operation in Windows, or perform a zero-filling technique, which improves SSD performance.
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More detailed information about the Optimize option for SSDs can be found at in our article on Defraggler and SSDs.
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https://www.ccleaner.com/docs/defraggler/technical-information/defraggler-and-ssds
It's an old debate as to whether SSD's should be defragged or not.
And the answer is: Generally not. (But there are exceptions).
SSD's don't realy need defragmenting, certainly not as much as a spinning disc HDD.
Because of the way that they work it doesn't (usually) matter if an SSD is fragmented, it will still be read from just as quickly as if it wasn't fragmented.
I won't get all technical, there's plenty of that online about the difference between SDD and HDD if you want technicals.
SSD's have a limit on the number of times that data can be written to them. (There again even HDD's have a 'lifetime' before they wear out, but that's much longer than early SSD's).
With early SSD's those limits were relatively low, and people were less aware of the differences between HDDs and SSDs. So the advice then was to <em>never</em> defragment them, to avoid 'using them up' unecessarily.
Newer SSD's have much higher 'lifetime' limits, and so an <em>ocassional</em> defrag would not have the same impact on lifetime.
(It's much the same reason why you shouldn't use Secure Deletion on an SSD, and shouldn't wipe the free space - to avoid unecessary overwrites. They are not needed on SSD's anyway).
If you have Windows 10 then that will do a very occasional 'inteligent defragment' of an SSD, as and when needed.
So I'd just 'Optimise' an SSD, if you want to, (or just leave it alone), and let Windows defragment it if/when needed.