Final answer:
Studying philosophy was not mentioned as a summer activity that Chris McCandless engaged in during his time off from college.
Step-by-step explanation:
The activity that Chris McCandless did not do during his summers off from college, as mentioned in the provided material, is c. Studying philosophy. The material provided describes his engagements in working an array of blue-collar jobs and pursuing intellectual interests on the side, but does not mention studying philosophy as one of his summer activities. The options given in the material reference different people and the various tasks they undertook, such as working on a farm or hitchhiking across the country; however, studying philosophy is not stated as one of his pursuits.