An experiment is the only way that psychologists can establish what kind of relationships through research? A. Positive relationships B. Negative relationships C. Correlational relationships D. Cause and effect relationships

"In psychological terms a relationship between variables means a connection between two factors that we can measure or scientifically vary.  Causal relationships are when one variable instigates a change in the other one, i.e. 'causing' this change to occur. Experimental research is used to investigate this sort of relationship, and discover whether or not one variable is really causing the change in the other.   
       
Moving on, another form of relationship is a 'Correlational' relationship. These can take the form of either a 'positive' relationship or a 'negative' relationship. A correlation is the measure of the relationship between to variable. A 'positive correlation' is where, as one variable increases, as does the other, and a 'negative correlation' is similar but, as one variable increases, the other decreases. However, there is no proof of a Causal relationship if a Correlational one is present; it merely hints that the two variables are related in some way.    

So, to answer your question, Causal relationships can only be proven through experimental research, but Correlational relationships can also be observed in an experiment but this does not prove anything, especially not a Causal link between the two variables.
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