What is a suspension?

A suspension is a heterogeneous mixture in which solid particles are dispersed in a liquid medium. In this type of mixture, the solid particles are large enough to settle out or be filtered out of the mixture. Therefore, the correct answer is "A solid in a liquid."

Option A, "A liquid in a solid," describes a colloidal dispersion where liquid is dispersed in a solid medium, which is not the definition of a suspension. Option C, "A gas in a liquid," describes a gaseous solution in a liquid medium, while option D, "A liquid in a gas," describes a liquid aerosol in a gaseous medium, neither of which defines a suspension.