Answer:
I believe the best choice is the first pair, "pat" vs. "pot".
Explanation:
As we know, a minimal pair is a pair of words that present only one different phoneme, that is, words that are not identical due to one single sound.
We are here trying to find a minimal pair related to low vowels. Low vowels are also called open vowels. They happen when the tongue is as far as possible from the roof of the mouth. For that reason, we could already choose "pat" and "pot" as the correct option. Pat /pæt / and pot /pɑt/ or /pɒt/ both present open vowels. The other pairs don't.
Still, what makes the pair "pat" vs. "pot" surely the right one is the fact that the phonemes æ and ɑ -- or ɒ, depeding if it is British or American -- are made different by their backness. Backness is the position of the tongue relative to the back of the mouth. In "pat", we have a front position, while in "pot" we have a back position.
