Answer : 3. "2, 8, 8, 2"
Explanation :

The electronic configuration of the element with atomic number 20 is 2, 8, 8, 2. 

The element with an atomic number of 20 is Calcium, Ca. 

The electron configuration is found using the Aufbau principle: 1s², 2s², 2p⁶, 3s², 3p⁶, 4s²

It belongs to the 2nd group and 4th period.