a
$(i)$ All the non$-$zero digits are significant.
$(ii)$ All the zeros between two non$-$zero digits are significant, no matter where the decimal point is, if at all.
$(iii)$ If the number is less than $1 ,$ the $zero(s)$ on the right of decimal point but to the left of the first non$-$zero digit are not significant.
$(iv)$ The power of $10$ is irrelevant to the determination of significant Figures. According to the above rules, $23.023$ has $5$ significant figures.
$0.0003$ has $1$ significant Figures. $2.1 \times 10^{-3}$ has $2$ significant figures.