According to significant figure convention the result obtained by adding $12.11,18.0$ and $1.012$ is

 Sum $=12.11+18.0+1.012=31.122$

Significant figure will be based on least number of digits after the decimal point.

$18.0$ has only one digit after the decimal point

So, the significant figure of $31.122$ is $31.1$

Therefore, option B is correct.