Final answer:
In the study by Schneider-Rosen and Cicchetti, socio-economic status was considered a confounding variable.
Step-by-step explanation:
In the study by Schneider-Rosen and Cicchetti, the researchers investigated the effect of maltreatment on toddlers' self-knowledge. In this study, socio-economic status was considered as a confounding variable.
A confounding variable is a variable that is related to both the independent and dependent variables, and it can influence the relationship between them. In this case, socio-economic status could potentially affect both maltreatment and toddlers' self-knowledge.
Therefore, the correct answer is C) A confounding variable.