In this process, a piece of paper with small holes in it separates a heterogeneous mixture by letting through the small particles and keeping out the big ones. That is called as

Answer: Filtration

Explanation:

Heterogeneous mixtures : It is a mixture that has non-uniform composition throughout the solution and the particle size or shapes are also different. There is a physical boundary between the dispersed phase and dispersion medium.

Homogeneous mixtures : It is a mixture that has uniform composition throughout the solution and the particle size or shapes are not different. There is no physical boundary between the dispersed phase and dispersion medium.

Filtration is a type of mechanical separation process in which the smaller particles are separated from the bigger particles through the filter paper where small particles pass through the fine holes of filter paper and the left ones remain on the filter paper.