Traci is working with a learner in her clinic. She has invited her learner to play with whatever toys they find in the therapy room. Janet has set out a number of stimuli for the learner to play with. She records what items the learner engages with and how long the engage with each item. This type of preference assessment is called: 

a) Naturalistic free operant observation

b) Contrived free operant observation 

c) Multplie stimulus without replacement 

d) Single-stimulus preference assessment