Aarush is tall but his brother is ________.

The correct answer is: Short

The correct answer is "short".
From the above-mentioned statement we understand that Aarush and his brother are described with opposite details because of the use of the conjunction "but". This "but" suggests that Aarush's brother will have the opposite feature of "tall". So, we must use the antonym of "tall" and the correct answer is "short".