It would be very helpful to have 3 arms. A person with 3 arms would be able to do so much more than a person with 2 arms. In the next 100 years will a human population will evolve to have 3 arms?

Impossible...

Explanation:

Members of say primate species required millions of years just to add a simple trait to their ongoing breeding line and for humans, a 100 years is mere short time. It is also impossible not because of time (Though we can manipulate genes) but naturally we are already highly diverse and adapted to most of the environments here on Earth. Say a third limbs would require a proper stimulus of need for at least over a million years to see such kind of evolution.

the answer for this question would be d. glaciers