Leleti believes that her friend spilled soda all over her backpack in order to get revenge for a remark Leleti made a few days ago, even though her friend claims that the incident was an accident. Leleti is making a:

causal attribution

Explanation:

Based on the information provided within the question it seems that Leleti is making a causal attribution. This term refers to the process of trying to determine the actual causes to an individual's specific behavior, in a personal or situational case. This is what Leleti is doing since, to her, the action of her friend spilling soda over her backpack is a personal action in order to get revenge.