To what extent do the levels of UVA, UVB and UVC radiations on the earth surface increase during solar flares?
During a powerful solar flare the amount of x-ray radiation can increase up to 110% in the boundry layer of the atmosphere/space,very little of this will reach the earths surface but it can be measured from high flying planes.

UVC is stopped by the upper layer of the atmosphere so this will never reach the ground,It might be possible to measure some UVC in antarctica because of the ozone hole but it is very weak so one will need special equipment to measure it.

UVB will change very little during a solar flare and only in the shortest wavelenght of the UVB spectrum will additional radiation be measurable.

UVA radiation will not change at ground level.

During a solar flare only the shorter wavelenghts of the electromagnetic spectrum will be affected,hard x-rays the most and the longer wavelenghts will be less affected.

source https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2005JA011507

source for more detailed information https://en.wikipedia.org/wiki/Ultraviolet



i do not have the source for this picture but i think it is free to use.