Final answer:
The main conclusion of the argument is that it proposes a specific course of action, which is to become a critical thinker and adopt the most defensible philosophical position.
Step-by-step explanation:
The main conclusion of the argument as presented in the provided question seems to be best summarized by option (c): The argument proposes a specific course of action. This can be inferred from the text which states the intent of the work is to encourage readers to become critical thinkers and to make the best informed decisions regarding the positions they choose to hold. The argument also suggests that some positions are better defended than others, driving towards a conclusion that endorses adopting the most defensible position. These points indicate that the argument is not merely presenting multiple perspectives or questioning existing theories but is actively persuading the reader to take a specific stance.