According to the law of constant proportions, the compound in which hydrogen and oxygen are present in a fixed mass ratio of 1:8 is:

The correct option is Water.

According to the Law of Constant Proportions (also known as the law of definite proportions), a chemical compound always contains its component elements in a fixed ratio by mass, regardless of its source or method of preparation.

Let us analyze the chemical formula of water, which is $H_{2}O$. A water molecule is composed of two hydrogen (H) atoms and one oxygen (O) atom.

The atomic mass of hydrogen is approximately 1 unit, and the atomic mass of oxygen is approximately 16 units.

To find the ratio by mass of hydrogen to oxygen in water, we calculate the total mass of each element in the compound:
Total mass of Hydrogen = 2 atoms × 1 unit = 2 units
Total mass of Oxygen = 1 atom × 16 units = 16 units

Now, we find the ratio of their masses:
Ratio of Mass of Hydrogen to Mass of Oxygen = 2 : 16

Simplifying this ratio by dividing both terms by their greatest common divisor, which is 2, we get:
Ratio = 1 : 8

Thus, in water, hydrogen and oxygen are always present in a fixed mass ratio of 1:8, verifying that the correct compound is Water.