Given:
A company wants to select 1 project from a set of 4 possible projects.
Consider the options are:
a.
b.
c.
d. 
To find:
The constraints that ensures only 1 will be selected.
Solution:
It is given that the company wants to select 1 project from a set of 4 possible projects. It means the sum of selected projects must be equal to 1.

Therefore, the correct option is (a).
The answer is D. A function is to where a line can be drawn upon the y axis and not touch two points.
This is the symmetric property, for you divide 9 to both sides to get x, instead of adding (addition property of equality)
So symmetric property should be your answer
Hope this helps