A year later, Alberto sets up an experiment where each participant is put in multiple conditions. For example, each participant tries to answer arithmetic problems while listening to metal, and then country, and finally classical music. Which design is he using now?

Within-Subjects Experiments

Explanation:

Within-Subjects Designs: this is a form of experimental design in which the participants involved is exposed or subjected to all the conditions.

However, in this case, Alberto, is using Within-Subject Designs (simultaneous) by exposing all the participants to answer arithmetic problems while listening to metal, and then country, and finally classical music simultenously.

The multiple conditions here are: arithmetic problems to solve, listening to different types of music in a sequence that mixed the different types.