D

Step 1: Understanding the photoelectric effect involves studying how light interacts with electrons in a material. When light rays hit a photo cell, it can cause the emission of electrons if the energy of the light is sufficient (i.e., if the light has a wavelength short enough to overcome the work function of the material).
Step 2: The time taken for electrons to be emitted after the incident light strikes the surface of the photo cell is extremely short. Typically, this process occurs in the order of picoseconds (10^-12 seconds) to nanoseconds (10^-9 seconds). However, in the context of the given options, we are focusing on slightly longer scales.
Step 3: Among the options provided (10^-1, 10^-3, 10^-6, and 10^-8 seconds), 10^-8 seconds is the closest to the known time frame for electron emission in photo cells. This aligns with known studies showing that photoelectron emission takes place within nanoseconds, but can be categorized as occurring on the scale of 10^-8 seconds after the incident of light rays.
Therefore, the answer is Option D: After 10 –8 sec on incident of light rays.