A.I. Explanation:

Option 1: "Personalised workout plans"

Explanation: While personalised workout plans are a feature of FitQuest, they are not the unique feature that specifically keeps users engaged and motivated. The text highlights that the app adapts to users' levels but does not emphasize this aspect as a motivational tool.

Option 2: "Nutritional advice"

Explanation: Nutritional advice is indeed part of FitQuest's offerings, but it is not identified as the unique feature that engages users. The text does not link nutritional advice directly to user motivation or engagement.

Option 3: "Progress tracking"

Explanation: Progress tracking is a valuable feature of FitQuest, yet it is not the unique aspect that keeps users motivated. The text mentions it as part of the app's functionality but does not connect it to user engagement in a distinctive way.

Option 4: "Challenge Mode"

Explanation: The 'Challenge Mode' is explicitly mentioned in the text as a unique feature that invites users to participate in daily challenges, earn points, and compete with friends. This aspect is directly linked to keeping users engaged and motivated, as it fosters a sense of community and competition.