Answer
$3.0\,\mu g$
Work Step by Step
Mass of water= density$\times$volume$=1.00\,g/mL\times100.\,mL=100\,g=0.1\,kg$
$30\,ppb\,Se\,in \,water= \frac{30\,\mu g}{1\,kg\,water}$
$\implies$ mass of Se in $\mu g$=$\frac{30\,\mu g}{1\,kg\,water}\times0.1\,kg$
$=3.0\,\mu g$