In the famous marshmellow test, children are given a marshmellow. If they successfully wait alone for the researcher to return, they are given another marshmellow. What skill does this illustrate?

delayed gratification

Explanation:

Delayed gratification describes the process whereby a subject resists the temptation of an immediate reward in preference for a later reward. Generally, delayed gratification is associated with resisting a smaller but more immediate reward in order to receive a larger or more enduring reward later.