If L=2.331 cm, B=2.1 cm, then L+ B= ?

To find the sum of the two measurements $L$ and $B$ with correct regard to significant figures, we follow the standard rules for addition and subtraction of measured quantities.

Given:
Length, $L=2.331\text{cm}$ (which has three decimal places)
Width, $B=2.1\text{cm}$ (which has one decimal place)

First, let us calculate the sum directly:
$L+B=2.331+2.1=4.431\text{cm}$

According to the rules for significant figures in addition, the final result must be rounded to the same number of decimal places as the measurement with the fewest decimal places.
Since $B$ has only one decimal place, the final sum must be rounded to one decimal place.

Rounding $4.431$ to one decimal place gives $4.4\text{cm}$.

Therefore, the correct answer is 4.4 cm.