The article's answer—tomato—is correct if the question is using botanical (scientific) definitions. Here's the breakdown:

 

So scientifically, tomato is the odd one out because it is a fruit, while the others are vegetative plant parts.

However, this is why the question is considered "tricky":

In everyday cooking

All four are often treated as vegetables:

• Tomatoes go in salads, sauces, and soups.
• They are usually used in savory dishes.
• Grocery stores often group them with vegetables.

In botany

A fruit is a structure that develops from a flower and contains seeds.

That means many foods people casually call vegetables are actually fruits, including:

• Cucumber
• Pumpkin
• Bell Pepper
• Zucchini
• Eggplant

A more challenging version of the puzzle would be:

Which one is not a root?

A. Carrot
B. Beetroot
C. Radish
D. Potato

In that case, the answer is potato, because potatoes are tubers (modified stems), while the others are roots.

So the article's answer is correct, but it relies on switching from the everyday culinary meaning of "vegetable" to the scientific botanical meaning of "fruit." That's what makes it a classic trick question.