As far as I can tell, a senior citizen only needs to have Medicare Part A (the "free" part) to avoid the Obamacare penalty.
I only have Part A, so I did search for the answer. The answer is that you only need part A.
I think it's AMAC, the alternative-conservative retirees organization, that had an answer to that question.
About Part B. A lot of people think that they have to have it and pay that monthly premium. But not so. The way it's done is a retiree has to opt in to part D but opt out of part B.