The least electropositive element among the following is:
A. $Na$
B. $Be$
C. $Li$
D. $Mg$

Hint: We can use the position of elements in the periodic table for predicting their properties, difference or similarities between them or to deduce a trend/order.We can understand the electropositive nature as ease of losing valence electrons.

Complete answer
After much effort and developments, we arrived at the modern periodic table which has proven to be quite useful in many ways. In order to have a better understanding of how it can be used, we have to know a few basic things. Let’s have a brief look at these:
The table is in the form of rows and columns that are known as periods and groups.
Elements are arranged as per their atomic number such that in a given period, all the elements have same outermost shell and in a group they have same number of valence electrons
This arrangement has given to some general trends of properties across period or in a group
Here, we have to consider the electropositive nature of the elements in a group and across a period. We can understand the electropositive nature as ease of losing valence electron. As it is known that size of elements increase down a group with increase in number of shells and decrease across a period as electrons get added in the same shell. It means that the distance between the nucleus and the valence electrons increases down a group and they can be given up easily so as we move down a group, electropositive nature increases. We can also see on the similar lines that the electropositive nature would decrease across a period. So, the element on the leftmost and the bottom-most would be highly electropositive whereas the element on the rightmost and top-most would be least electropositive.
Here, we have four elements, $Na,Be,Li\,{\rm{and }}\,Mg$ whose arrangement in the periodic table can be represented as follow:

As it is evident that among the given elements, one which is on the rightmost and top-most is $Be$.

Hence, the correct option is B.

Note:
It is useful to remember that we have electropositive metals on the left side of the periodic table whereas electronegative non-metals on the right excluding the noble gases. So both the metallic character and the electropositivity of the elements follow this general trend.