Concept:Use a reciprocal pronoun to show that two people share the same action towards each other.
Explanation:Since the sentence names two people, Mary and Jane, the correct phrase is "quarrelling with each other."
"Themselves" would mean each person quarrels with herself, which does not make sense here.
"Another" is used with a singular noun, so it does not fit the pair mentioned.
"Either" refers to choosing one of two options, not to a mutual action.
Therefore, the only suitable choice is "each other."
Answer:A. each other