This is general advice and not specifically for you:
That is why it shouldn't be used unless recommended and then only under advice from someone experienced in its use and the information it produces.
It could seriously impact on your system should you chose options were you don't know what the impact might be.
The unknown MBR could mean more than one thing and not always malicious. It could be an indication that malware has modified the MBR code, but you would likely be experiencing other symptoms. Perhaps more commonly this could be because of the system that you have, Dell, Acer, etc. where they have got a manufacturers recovery console and recovery partition.
To achieve that they have to customise the MBR record, if anyone chose Fix in this instance they would be wiping that custom MBR code and would lose access to that recovery console.
So care has to be exercised when using tools such as these as that may return information which could be incorrectly acted on.