Answer
3245 ug is the greater mass.
Work Step by Step
3245 ug = 3245 * $10^{-6}$g = $3.245 * 10^{-3}$ g
0.00515 mg = 0.00515 * $10^{-3}$ g = 5.15 * $10^{-6}$g
$3.245 * 10^{-3}$ g is bigger than 5.15 * $10^{-6}$g, so 3245 ug is the greater mass.
