Generally, the atomic radius increases as we move down a group due to the addition of new electron shells. However, in Group 13, the atomic radius of Gallium (Ga, 135 pm) is smaller than that of Aluminium (Al, 143 pm). This anomaly occurs because Gallium has 10 completely filled d-electrons in its inner shell. These d-electrons provide a very poor shielding effect compared to s and p electrons. As a result, the effective nuclear charge felt by the outermost electrons increases, pulling them closer to the nucleus and decreasing the atomic radius. Therefore, the correct order of increasing atomic radii is B < Ga < Al < In < Tl.