Extravagant is about how much is spent. Ostentatious is about how much it is shown off.
Both words suggest something over the top, so children treat them as the same criticism. One is about the cost, the other about the audience.
Extravagant means spending far more than you need. Buying three new football kits when one would do is extravagant, even if they stay folded in a drawer and nobody ever sees them.
Ostentatious means showing off so that people notice. A birthday cake decorated with gold-coloured icing and a giant photo of the birthday boy, carried round every table so everyone has to admire it, is ostentatious. Something can be ostentatious without costing much at all: the point is the showing, not the price.
Spending or costing far more than is needed.
Similar: wasteful, fancy, showy
Showy in a way that is meant to impress other people.
Similar: showy, flashy, extravagant
Comprehension answers often need you to explain why a character does something. Ostentatious tells you the character wants to impress others, while extravagant only tells you they spent too much. Choosing the right one shows you have understood the motive.
Definitions and example sentences on this page are the ones a child sees inside Wordrealm, taken from our list of over 1,100 words. It is free to browse, with nothing to sign up for.
Plain-English guides to each test, from the councils’ own pages: Kent Test · Medway Test · Bucks Secondary Transfer Test