A dictionary is arranged in alphabetical order. It seems as though our mental lexicon is organized primarily according to _____.

The mental lexicon, which refers to the mental store of words and their meanings, is primarily organized by frequency. Research in cognitive psychology suggests that the words we use and recognize are influenced significantly by how often we encounter them in our daily lives. This frequency-based organization allows individuals to retrieve words more efficiently when needed since the most frequently used words are more readily accessible in the mental lexicon.

In contrast, other options pertain to different concepts within cognitive psychology. The invariance problem relates to the challenge of recognizing objects or words regardless of variability in perception. The heir approach does not have a direct application in the context of word retrieval or lexicon organization. The prototype approach deals with categorization, where concepts are understood in relation to an idealized or typical representation rather than based on frequency of usage. Thus, frequency is the most appropriate choice for describing how our mental lexicon is organized.