Which of the following is a symptom of an oil leak?

A drop in oil level is indeed a strong indicator of an oil leak. When there is a leak, the oil that is supposed to lubricate the engine components escapes from its designated spaces, resulting in a lower oil level as measured on the dipstick. This can lead to insufficient lubrication, which increases the risk of engine damage and reduces overall performance.

In contrast, an increase in oil viscosity typically suggests that the oil is breaking down or becoming contaminated with dirt and combustion byproducts, rather than indicating a leak. Engine knocking noise can be a symptom of various mechanical issues, including low oil pressure, but it's not directly tied to the presence of an oil leak itself. Lastly, fuel dilution in oil, while indicative of other problems such as fuel entering the oil system, does not confirm an oil leak specifically. Therefore, a drop in oil level is the most direct and recognizable symptom related to an oil leak.