Relationships among people are
a. one of two types—social or working.
b. usually simple and easy to understand.
c. not influenced by people's ideas and attitudes.
d. something that must be maintained to stay healthy and strong

Let's look at all the options:

A. one of two types—social or working.
-there also also other kinds, for example family relations!

B. usually simple and easy to understand.
-this is not true. If it was true, there wouldn't be any inter-personal conflicts!

C. not influenced by people's ideas and attitudes.
-they are very influenced by people ideas! in fact, the opposite is true!

D. something that must be maintained to stay healthy and strong
-this is the correct answer! if the relations are not maintained, they will fade .