Rats that received unpredictable electric shocks in a laboratory experiment subsequently became apprehensive when returned to that same laboratory setting. this best illustrates that anxiety disorders may result from:

Classical conditioning

Explanation:

Classical or respondent conditioning is a procedure for learning in which a biologically potent stimulus is combined with a neutral stimulus and in this process, the neutral stimulus is known to exert a response that is similar to the one that is caused by the biologically potent stimulus. 

Here, the unpredictable electric shocks classically condition the rats to be apprehensive of the shocks.