Answer:
The answer is d.
Explanation:
This concept refers to adapting to an environment by blocking sensorial distractions. It can apply to all senses (i.e. you get used to a smell, sound or visual stimulus to the point at which you ignore it).
For example, while taking an exam in a classroom, the sound of a ticking clock on the wall might be very distracting at first. In time, your brain will automatically filter the sound so that you can concentrate.