Which assessment represents a subjective measure of motivation?

A subjective measure of motivation focuses on personal perceptions, feelings, and opinions, rather than objective, quantifiable data. In this case, participants' answers on the exit questionnaire represent their personal reflections and insights regarding their motivation during the task, making it a subjective assessment. Participants can express their level of interest, enjoyment, and engagement in their own words, which captures the nuances of their internal motivation that cannot be quantified by measurable actions alone.

The other choices involve more objective measures. The number of grid sequences attempted and the time spent not playing the game provide tangible data on behavior but do not capture the participants’ internal states. The decision to continue playing or stop to read a magazine may suggest motivation but does not provide direct insight into the participants' thoughts or feelings like the questionnaire does. Hence, the exit questionnaire is the clearest reflection of subjective motivation.