Awww, you knew it was a trick question – right – the minute you saw the tomato? (Yes, that purple blob on the right is a Purple Russian.) Everybody knows a tomato is a fruit, not a vegetable.
In this case, these are all fruits: Osage Orange (inedible by humans), pepper, cucumber and tomato. The basic rule of thumb is that a fruit is a seed-bearing plant component, and a vegetable is a root (carrot), tuber (potato), stem (celery), bulb (onion), flower (broccoli) or leaf (lettuce). In short, a fruit is the mature ovary of a plant.
That explains why the same plant family can have both fruits and vegetables. For example, the Solanaceae family has fruits (peppers, tomatoes) and vegetables (potato). Let’s not complicate matters by mentioning “seed potatoes” – that’s just a figure of speech.
As a real stretch, what about green beans (string beans)? They have seeds inside, so are they a fruit? Technically yes, as are all legumes.