Choose the incorrect trend in the atomic radii (r) of the elements :

Atomic radius decreases from left to right across a period because nuclear charge increases while the principal quantum number $$n$$ remains the same.
Atomic radius increases down a group because a new shell is added, so the outer electrons are farther from the nucleus despite the higher nuclear charge.

We now test each option one by one.

Case A: $$r_{Br} \lt r_{K}$$
Both Br and K lie in the 4th period. K is in Group 1 and Br in Group 17. Moving from Group 1 to Group 17 in the same period, atomic radius must decrease. Hence $$r_{Br} \lt r_{K}$$ is a correct trend.

Case B: $$r_{Mg} \lt r_{Al}$$
Mg and Al are neighbours in the 3rd period (Mg: Group 2, Al: Group 13). Across a period, radius should decrease, so $$r_{Mg} \gt r_{Al}$$ in reality. The given inequality reverses this order, so it is incorrect.

Case C: $$r_{Rb} \lt r_{Cs}$$
Rb and Cs belong to Group 1. Going down the group from Rb (5th period) to Cs (6th period), radius increases, i.e. $$r_{Rb} \lt r_{Cs}$$. This statement is correct.

Case D: $$r_{Al} \lt r_{Cs}$$
Al is in the 3rd period, Cs in the 6th period. Down a group and also across many periods toward the left, Cs becomes much larger than Al, therefore $$r_{Al} \lt r_{Cs}$$ is also correct.

Only Case B shows an inequality opposite to the actual periodic trend, so Case B (Option B) is the incorrect trend.

Final answer: Option B.