The correct order of atomic radius of Li, Na, Be and O is

Atomic radius increases down a group and decreases across a period. Na (Period 3) has the largest radius. Li and Be are in Period 2; Li (Group 1) is larger than Be (Group 2) because radius decreases across a period. Oxygen is further right in Period 2 and has the smallest radius among these. Therefore the correct order is Na > Li > Be > O.