Three compounds that nitrogen and oxygen can form are NO, NO2, and N2O3. The law of multiple proportions tells you A. the ratio of nitrogen to oxygen is always the same.
B. nitrogen combines with oxygen in small whole number mass ratios.
C. energy is conserved.
D. mass is conserved

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.

the answer to your question is c