My answer is (c).
But, I believe I can give some perspective why many would answer (a). First, geography is a big issue for me, as it is for a lot of post-bacc'ers. Most DO schools tend to be located in certain regions of the country, and many are in rural areas. Yes, it's true that residency location is more important than the location of your medical school. But, many graduates do end up doing residency near their medical school. So, medical school location is important. I, for one, have no interest in attending medical school or practicing in a rural area.
Second, there are additional hassles one must endure as a DO student. For one, there is a separate licensing test, the COMLEX. While widely accepted by allopathic residency programs, some specialty residencies prefer to see the USMLE scores. So, many DO students endure two licensing exams. Setting up clinical rotations is more of a hassle at some DO schools as well.
Finally, many DO schools push themselves as being "primary care" focused, or even "rural primary care" focused. While that all sounds nice and makes everyone feel warm and fuzzy, the fact is that many incoming students are not necessarily dedicated to becoming primary care docs upon matriculation. Personally, I am undecided as to specialty, etc., and prefer to keep my options open. Yes, I do realize that there are many practicing DO's in all medical specialties. But I think the DO schools are doing themselves a disservice here by unnecessarily scaring off some well qualified potential applicants.
BTW, I have nothing against DO's or DO schools. The above is just an impression I get from my research into this matter. I don't think any of the above (except, maybe, for geography) should keep applicants from exploring the DO route.