The reaction between X and Y, forms compound Z, X loses an electron and Y gains an electron which of the following properties is NOT shown by Z? 

Ionic bonds, also known as electrovalent bonds, are created when one metallic atom loses electrons and another non metallic atom obtains electrons. A new bond develops between two oppositely charged ions due to electrostatic attraction between them. And an ionic compound is one that has developed as a result of ionic bonding.

In this question, it is stated that X loses an electron, causing it to behave as a metal  (electropositive element), whereas Y gains an electron, causing it to behave as a non-metal(electro-negative element). As it is well known, an ionic bond or electrovalent bond is created when two charges that are opposite to one another come together due to attraction. 
The strongest of all bonds are ionic bonds. Ionic bond molecules have a high melting and boiling point as ions are bound very closely to each other. In a molten form or in an aqueous solution, molecules with ionic bonds make good electrical conductors due to presence of oppositely charged ions. 

Hence, the correct answer is option B.