Video solution:

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Written solution:

We represent the lengths of each square as $s_1,s_2,$ and $s_3$ respectively. The length of the rectangle is $s_1+s_2+s_3$ as these $3$ squares span the entirety of a side of the large rectangle. Therefore, $$s_1+s_2+s_3=3322.$$

Also, the height of the large rectangle is the sum of the height of $R_2$ and $S_3.$ Now, note that the sum of height of $R_2$ and $s_2$ is $s_1,$ so height of $R_2$ is equal to $s_1-s_2.$ Therefore, the height of the large rectangle is $s_1-s_2+s_3,$ which means $$s_1-s_2+s_3=2020.$$ Subtracting both of our results yields $$2s_2=3322-2020=1302.$$ This would mean $$s_2=651.$$

Thus, the correct answer is A.