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 .