Answer
Please see the work below.
Work Step by Step
We know that
a. This model contains three atoms of two different elements that is H and O. Hence, the model is $H_2O$.
b. The model represents a crystal which contains two different elements in a 1:1 ration. Thus , the given model represents ionic compound KCl.
c. The given model has six atoms, four of which are the same H and two others C and O. Hence, the model shows $CH_3OH$.
d. The given model has four atoms of two different elements N and H. Hence, the model shows $NH_3$.
