Use periodic trends of atomic radius

Atomic radius generally:

• increases down a group
• decreases from left to right across a period

Check each option

Option A: $r_{\mathrm{Rb}} < r_{\mathrm{Cs}}$

Rb and Cs are in the same group, and Cs is below Rb. So radius increases down the group: $r_{\mathrm{Rb}} < r_{\mathrm{Cs}}$ Hence, A is correct.

Option B: $r_{\mathrm{At}} < r_{\mathrm{Cs}}$

Cs and At are in the same period (period 6). Across a period, atomic radius decreases from left to right. Cs is far left, At is far right. Therefore: $r_{\mathrm{At}} < r_{\mathrm{Cs}}$ Hence, B is correct.

Option C: $r_{\mathrm{Br}} < r_{\mathrm{K}}$

K and Br are in the same period (period 4). Across the period, radius decreases from K to Br. Thus: $r_{\mathrm{Br}} < r_{\mathrm{K}}$ Hence, C is correct.

Option D: $r_{\mathrm{Mg}} < r_{\mathrm{Al}}$

Mg and Al are in the same period (period 3). Across a period, radius decreases from left to right. Since Mg is to the left of Al: $r_{\mathrm{Mg}} > r_{\mathrm{Al}}$ So the given trend $r_{\mathrm{Mg}} < r_{\mathrm{Al}}$ is incorrect.

Conclusion

The incorrect trend is: $\boxed{D}$

Comparison with stored correct answer

Stored correct answer: $D$

My derived answer also is $D$, so they agree.