Parsimony is a guiding principle in science including Applied Behavior Analysis which states that when multiple explanations for a phenomenon are possible the simplest explanation that requires the fewest assumptions should be preferred It encourages practitioners to favor explanations that are direct observable and testable over those that are more complex rely on unobservable constructs or involve multiple unverified inferences In the given options the explanation that Alex stopped participating because he felt unwell and was experiencing a headache Option A proposes a single clear and relatively direct reason This explanation involves one primary assumption that can be directly verified eg by asking Alex or observing physical signs Options B C and D on the other hand introduce multiple often convoluted or less verifiable assumptions Option B Introduces multiple reasons low grade discouragement argument sadness withdrawal making it less parsimonious than a single cause Option C Relies on abstract untestable and nonbehavioral concepts like personal vendetta negative energy past life trauma and Mercury in retrograde which introduce numerous unsubstantiated assumptions and are outside the scope of behavioral analysis Option D Combines a moodrelated cause favorite sports team lost with an environmental factor affecting cognition faulty AC again introducing more assumptions than necessary Therefore Option A is the most parsimonious because it offers the simplest explanation with the fewest unsupported assumptions