Alejandro is developing a preliminary research study to address the question of whether 4-year-olds who are asked to delay gratification (e. g., to delay eating candy) perform differently in solitude than with other 4-year-olds. which combination of research perspectives would be best for this preliminary study?

The answer is "Developmental Psychology and Social Psychology".

Developmental psychology is the scientific study of how and why individuals change through the span of their life. Initially worried about babies and youngsters, the field has extended to incorporate immaturity, grown-up improvement, maturing, and the whole life expectancy.  

Social psychology  is the scientific study of how individuals' musings, sentiments, and practices are affected by the real, envisioned, or suggested nearness of others.

sounds good to me mate!