Forgetting to remove the telescope lens cap is not usually the problem.
The answer to this multiple choice question is "red shift" which is actually a useful phenomenon in viewing and analyzing the light from very distant stars. Light is shifted in wavelength by the movement of stars toward or more typically AWAY from the observer, or by moving through strong gravitational fields.
Nebula, white dwarf, red giant, black hole
star of one stellar mass, red giant, planetary nebula, white dwarf
They don't understand how redshift affects stars.