I am developing a Software Project Plan and I am referencing a template that seems to define the Purpose section as describing the purpose of the project plan, and not the purpose of the product for which the project plan is being developed.
Without seeing the template you're referencing, I expect it's like the introduction section of a lab report: the introduction section introduces the report not the experiment that the report is about.
The Software Project Plan will have a different purpose from the product you're making. And the purpose of the product you're making should be defined in a requirements document of some description not the plan. Otherwise there is too much likelihood of contradictions.
So, to directly answer your question, the Software Project Plan's Purpose section should be about the purpose of the plan (option A): Are you time-limited? Are you resource-limited? Are you financially constrained? What assumed constraints was this plan produced under?