user
I am aspired by all, yet hard to find.  You can search endlessly, but have little peace of mind.  What am I?
A: exploration
B: activity
C: state
D: happiness
E: piece
assistant
<|begin_of_thought|>

Deep Thought is thinking for 7.5 million years.

<|end_of_thought|>

<|begin_of_solution|>

D

<|end_of_solution|>