The initial and final temperatures of a water bath are (18 ± 0.5)°Cand (40± 0.3)°C. What is the rise in temperature of the bath ?

To find the rise in temperature of the water bath and the associated error, we follow these steps:

Let the initial temperature be $T_1=(18\pm 0.5)\text{°C}$ and the final temperature be $T_2=(40\pm 0.3)\text{°C}$.

The rise in temperature, $T$, is the difference between the final temperature and the initial temperature:
$T=T_2-T_1$
$T=40\text{°C}-18\text{°C}=22\text{°C}$

When subtracting two quantities, their absolute errors are always added together to find the maximum possible error in the result:
$\Delta T=\Delta T_1+\Delta T_2$
$\Delta T=0.5\text{°C}+0.3\text{°C}=0.8\text{°C}$

Therefore, the rise in temperature of the bath is:
$T\pm \Delta T=(22\pm 0.8)\text{°C}$

Thus, the correct option is (22 ± 0.8)° C.