Which of the following is used to separate substances based on density in a sample?

The correct answer identifies centrifuge tubes as the apparatus used to separate substances based on density in a sample. Centrifuge tubes are designed to be used in a centrifuge, which spins the tubes at high speeds. As the tubes spin, the centrifugal force pushes denser substances to the bottom of the tube, creating layers within the mixture. This process allows for the separation of components based on their differing densities, making it a useful technique in various biological and chemical applications.

The other options do not serve the function of separating substances by density. A hot plate provides heat for processes like boiling or heating substances but does not separate them. A triple beam balance is used for measuring mass, which is different from separating components in a mixture. Dissecting scissors are tools used for cutting and dissection, with no relevance to separation based on density. Thus, centrifuge tubes are the correct choice as they directly facilitate the separation of substances through the application of centrifugal force.