The correct answer is C. 4 parts of acid to 6 parts of water.
The specific gravity of a substance is its density relative to that of water. Water has a specific gravity of 1.0, so a substance with a specific gravity of 1.8 is 1.8 times as dense as water.
To get a mixture of specific gravity 1.3, we need to add water to the sulfuric acid until the average density of the mixture is 1.3. The specific gravity of water is 1.0, so we need to add 0.3 parts of water for every 1 part of sulfuric acid.
Therefore, the ratio of acid to water to get a mixture of specific gravity 1.3 is 4 parts of acid to 6 parts of water.
Option A is incorrect because it would result in a mixture with a specific gravity of 1.2.
Option B is incorrect because it would result in a mixture with a specific gravity of 1.4.
Option D is incorrect because it would result in a mixture with a specific gravity of 1.6.