Two elements A and B belong to the 3rd period of the modern periodic table and are in groups 2 and 13, respectively. Correct comparison of their tendency to lose electrons is:

Two elements A and B belong to the 3rd period of the modern periodic table and are in group 2 and 13, respectively. The correct comparison of their tendency to lose electrons is A > B.

Upon moving from left to right in the modern periodic table, size of the elements decreases because effective nuclear charge on the outer shell increases without an increase in the number of shells. So, it becomes harder to lose electrons. 

Hence, A has a greater tendency to lose valence electrons to attain noble gas configuration than B.