In this question, we are given an assertion (A) and a reason (R) related to carbon dioxide.

Assertion (A) states that carbon dioxide is a compound, which is true. Carbon dioxide is a chemical compound made up of one carbon atom and two oxygen atoms.

Reason (R) states that carbon and oxygen combine together in the ratio 3:8 by mass, which is also true. The molar mass of carbon is 12 g/mol, and the molar mass of oxygen is 16 g/mol. Therefore, the ratio of mass between carbon and oxygen in carbon dioxide is 12:32, which simplifies to 3:8.

Now, we need to determine the relationship between the assertion and the reason.

Option 1 states that both A and R are individually true and R is the correct explanation of A. This is the correct answer. The reason given for the assertion accurately explains why carbon dioxide is a compound. The combination of carbon and oxygen in the given ratio by mass results in the formation of carbon dioxide.

Therefore, we can conclude that carbon dioxide is a compound, and the given reason explains why.