1) Do you know what is a tertiary carbon?
They are carbons attached to othe three carbons
2) Do you know the structue of methylcyclopentane?
It is a cyclopentane molecule with a CH3- group attached to one carbon
3) Do you know which carbons are tertiary in methyl cyclopentane?
The only tertiary carbon in methyl cyclopentane is the carbon to which the methyl (CH3) group is attached.
4) So, there is just one monofluoro derivative of methylcyclopentane with the fluorine on a tertiary carbon: it is the 1-fluoro-1-methyl-cyclopentane.
I also attach the drawing