We need to find the fewest number of significant figures in the given options.
A. 360.01 has five significant figures.
B. 5,003.1 has five significant figures.
C. 296.54 has five significant figures.
D. 18.3 has only three significant figures.
Therefore, the fewest number of significant figures are 3.
Therefore, correct option is D option.
D. 18.3.