A series is given, with one number missing. Choose the correct option from the given ones that will complete the series.
$-10.8$, $-8.9$, $-7$, ?, $-3.2$

Let us find the common difference between consecutive terms in the series:
* $-8.9-(-10.8)=-8.9+10.8=+1.9$
* $-7-(-8.9)=-7+8.9=+1.9$

Since the common difference is $+1.9$, the missing term can be calculated as:

$$\text{Missing Term}=-7+1.9=-5.1$$

Let's verify with the next term:

$$-5.1+1.9=-3.2$$

This matches the final term. Thus, the missing number is $-5.1$.