Answer:
b. micro expression.
Step-by-step explanation:
Micro expression is a facial expression which occurs in a flash and is hardly noticeable by others. Most of the times this expression occurs involuntarily and is said to expose our truest emotions.
It is barely noticeable yet it occurs to everyone and cannot be controlled. This is why some people though they keep on smiling and laughing, we can still feel that there is something hidden in their outwardly expressions.