I took Psychology Today's Classical IQ test(https://www.psychologytoday.com/intl/tests/iq/classical-iq-test) and encountered this question that I wasn't able to solve:
In fact, this is just the first question of the 57 questions in the test. Although I can just click the <I don't know> button and move on, I would really like to know what is the answer to this problem and the explanation to it. Any help in this problem is appreciated. Thank you.