an oxymoron is a figure of speech where two words are put together but they actually contradict (or go against) one another.
a. (common rarity) is an oxymoron because something cannot be both common and rare at the same time.
c. (proudly humble) is also an oxymoron because proud and humble are antonyms as to be humble means to show a modest or low estimate of one's own importance.
hope this helps!
