A compound is formed when two or more elements are chemically combined in which type of proportion?

Fixed proportion

The correct answer is option 2. Fixed proportion.

A compound is formed when two or more elements are chemically combined in a Fixed proportion.

Compounds are distinct from mixtures because the elements in a compound are always present in a specific, definite ratio by mass. This is governed by the Law of Constant Composition, also known as Proust's Law.

Here's why the other options are not correct:

Variable proportion: This describes mixtures, where the components can be present in varying amounts.

Either Variable or Fixed proportion: Not all combinations of elements form compounds. If a compound is formed, the proportions will be fixed.

None of these: This is incorrect because elements in a compound do have a specific way they combine.