a
Here,
$\mathrm{t}_{1}=20^{\circ} \mathrm{C} \pm 0.5^{\circ} \mathrm{C}$
$\mathrm{t}_{2}=50^{\circ} \mathrm{C} \pm 0.5^{\circ} \mathrm{C}$
The temperature difference of two bodies is
$\mathrm{t}=\mathrm{t}_{2}-\mathrm{t}_{1}=50^{\circ} \mathrm{C}-20^{\circ}=30^{\circ} \mathrm{C}$
The error in temperature differerence is given by
$\Delta \mathrm{t}=\left(\Delta \mathrm{t}_{1}+\Delta \mathrm{t}_{2}\right)$
$=\left(0.5^{\circ} \mathrm{C}+0.5^{\circ} \mathrm{C}\right)=1^{\circ} \mathrm{C}$
The temperature difference is $30^{\circ} \mathrm{C} \pm 1^{\circ} \mathrm{C}$