Color is somewhat subjective, that is why people study wavelengths with atomic spectra rather than color. For elementary classes, this is fine. Certainly A is a wrong choice because I checked the transmission characteristics of cobalt glass,* the blue glass transmits both the violet and the deep red doublets of potassium. So, the flame color appears like lilac as you can see in various Youtube videos of flame tests with cobalt glass.
In low temperature flames like that of the Bunsen burner, K atoms emit 404.4, 404.7 (pure violet), and 766, 769 nm (really deep red, pretty hard to see in a pocket spectroscope). The resulting mix of emission color looks like the classical lilac potassium flame shown in textbooks.
*Ref: Frederick C. Strong III, Improving the Potassium Flame, J. Chem. Educ. 1969, 46, 3, 178 (behind paywall)