The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
The inert pair effect is the tendency of the electrons in the outermost atomic s orbital to remain unionized or unshared in compounds of post-transition metals. The term inert pair effect is often used in relation to the increasing stability of oxidation states that are two less than the group valency for the heavier elements of groups 13,14,15 and 16. Boron does not show univalent nature but unipositive nature of thallium is quite stable, because of Inert pair effect in thallium.