Which one of the following was the contribution of Hugo de Vries?

Natural selection can lead to . . . . . . .
$(a)$ stabilisation
$(b)$ genetic drift
$(c)$ directional change
$(d)$ disruption
Choose the correct answer from the options given below:
$(A)$ $(a)$, $(b)$ and $(c)$ only
$(B)$ $(a)$, $(c)$ and $(d)$ only
$(C)$ $(a)$, $(b)$, $(c)$ and $(d)$
$(D)$ $(a)$ and $(c)$ only

The concept that "Population tends to increase geometrically while food supply increases arithmetically" was put forward by:

Select the correct statement from the given options.

In natural selection, more individuals acquire value other than the mean character value, which indicates . . . . . . .

Who was the first scientist to estimate the human population?