At room temperature, a mixture of sand and water can be separated by
(1) ionization (3) filtration
(2) combustion (4) sublimation

The answer is (3) filtration.

Filtration is a process of separating solids from fluids. It is usually used for heterogeneous mixtures such is a mixture of sand and water. So, by adding a filtrate, through which a fluid can pass, solid particles are trapped in the filtrate. If you use a physical filter to separate water from sand, the sand will remain on the filter, while the water will pass through.

The correct answer is "hydrogen".