How many significant figures are there in each number? …

(a) $0.04050$: the zeros at the left do not count; $4$, $0$, $5$ and the last $0$ do, so there are $4$.

(b) $3.0800$: all digits count, because the zeros at the right follow a decimal point. There are $5$.

(c) $7000$: the zeros at the right of a whole number are not clear, so only $1$ is certain.

(d) $7.000 \times 10^{3}$: there are $4$. This is how to show that all four figures of $7000$ are meant.

(e) $0.0008$: only the $8$ counts, so there is $1$.