Which of these elements has the lowest melting point?
(1) li (3) k
(2) na (4) rb

The answer is (4) Rb. From observation, we can find that these four elements are from first column. The melting point from top to the bottom is from high to low. So the answer is (4).

Ido not understand. 

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).