Answer:
C. Ground water infiltration
Step-by-step explanation:
Ground water infiltration can cause wells on land to become saline making them unusable for drinking water.
When rain falls down, it will sip into the ground. It might encounter minerals (like salt) along its path and dissolve them until it reaches a reservoir.
That reservoir could be the well itself, but it's most likely it will be another natural reservoir nearby, in which the salt and minerals concentration will raise gradually... then that second reservoir will somehow connect with the well, contaminating its water.