A — The growth of faith
B — The growth and worldwide influence of Christianity
C — The growth of evil and corruption in the world
C best fits both the context of the surrounding parables and the historical background, and context is crucial.
The parable describes a mustard seed growing into a large tree. For people of that time, such growth could have a negative connotation. Mustard was known as an aggressive and invasive plant. The Roman writer Pliny the Elder (AD 23–79) observed: “Mustard… when it has once been sown it is scarcely possible to get the place free of it again, as the seed when it falls germinates at once.” (Natural History 19.171).
Rabbinic literature reflects a similar concern. The Mishnah (Tractate Kilayim) discusses mustard in the laws of mixed planting because it spreads easily and can invade other crops, creating forbidden mixtures (kilayim). Interestingly, the Hebrew word for mustard, ḥardal, can be viewed as combining har (“mountain”) and dal (“poor” or “weak”), while the Greek word sinapi may be related to a root meaning to sting or irritate.
For this reason, to avoid confusion with Matthew 17:20, where the mustard seed clearly symbolizes the growth of faith, this parable might better be understood as illustrating an abnormal and problematic growth—something small that expands and eventually dominates.
Note that this parable in Matthew 13 is part of a group of 7 parables. The first 4 depict a prophetic and problematic view of the Kingdom Age, showing the presence of both true and false believers and the development of an institutionalized faith that made it easy for false doctrine to enter.