We are asked to determine the smallest celestial body.
The universe is where all the celestial bodies are contained, therefore, this is the largest of the celestial bodies.
A nebula is a giant cloud of dust where the stars are formed, therefore, this is not the smallest of the celestial bodies.
A spiral galaxy is a galaxy in the form of a spiral. It contains a large number of stars and planets and other celestial bodies, therefore, this is not the smallest celestial body.
Our sun is a star and in comparison with the celestial bodies decribed in A, B, and C this is the smallest of the celestial bodies.
Therefore, the correct answer is D.