White, or lighter colored, shirt's reflect most of the visible wavelengths it is exposed to, a black or dark shirt absorbs more wavelengths. Absorbed radiation is converted into heat, making the darker shirt warmer to wear.
So it is cooler because I reflects light!
Hope I helped. :)
Answer: The correct answer is- Cooler because it reflects light.
White color is cooler in the sunlight because it does not absorb radiations coming from the sunlight. Instead, white color shirts reflect the light when it hits the body.
Due to this, heat is not produced in the body and we feel cool.
Thus, white shirt is cooler in the sunlight.