"Jina has been watering the plants since 5pm."
Explanation:
"Since" is used to indicate the starting point of an action that began in the past and continues to the present (e.g., "since 5pm").
"For" is used to show a duration of time (e.g., "for 2 hours").
In this case, since you're referring to the specific time when Jina began watering the plants, "since" is the appropriate word.
