In many humans exposing the skin to sunlight or a prolonged period of time results in the production of more pigment by the skins tanning this change in skin color provides evidence that

changes in the environment could influence gene expression.

Explanation:

In humans, skin color is defined by the abundance of "melanin" pigment. The darker is skin, high pigmentation is there. Studies have proven that high temperature is the major reason for high pigmentation/darker skin. This is due to the reason that high temperature leads to the increased expression of the specific pigmentation related genes (e.g. MC1R gene). So, if a human is exposed to high temperature for a longer period, the action (expression) of the MC1R gene will be high as well. This will result in a tanned skin color. This is the reason that those humans who live near to the equator (warmer climate) have darker skin whereas those humans that live close to poles (cold climate) have light-colored skin.

"do a testcross with a homozygous recessive plant."

b. lithosphere, asthenosphere, mesosphere   is the answer most likely