If $73_x$ (in base-x number system) is equal to $54_y$ (in base $y$-number system), the possible values of $x$ and $y$ are
Answer is D.
$x\times 7 + 3 = 5 \times y + 4 \implies 7x = 5y + 1$.
Only option satisfying this is D.
Thank you so much Arjun Sir!