a. something written to be more-or-less complete course in playing (e.g. not necessarily much theory) usable with preferably a tutor but maybe without.
b. normally understood to be an approach derived from a particular pedagogue who may well have published a range of materials - the Carlevaro School comes to mind. Could also be an institution, probably fairly local run by a tutor.
c. simply a set of targets and ways of measuring progress. Not a complete course in itself, though often used as the framework for such. Especially useful for finding progressive repertoire to pursue.
d. self taught!
The course is find the best tutor you can and follow their approach. It may well not cover absolutely everything but the regular input, guidance, encouragement and admonition is basically irreplaceable.
Most of the above are not 'versus' each other, most are complementary.
Simon Ambridge Series 40 (2005)
Trevor Semple Series 88 (1992)
Louis Panormo (1838)
Alexander Batov Baroque Guitar (2013)