Diseases such as a cold or flu are caused by

a. miasma
b. decaying matter
c. microorganisms
d. temperature changes

The correct answer is option C, microorganisms

Reason -

Common cold or flu are viral diseases and thus they are communicable. Being a viral disease, there must be a virus ( considered as a micro-organism) which is  responsible for spreading the disease. For instance common cold is caused by several viruses such as,  influenza virus, oronavirus, respiratory syncytial virus, and parainfluenza. These viruses spreads with any sort of physical contact with the infected person.