according to definition,

subset B covers subset A when you add only one element to subset A which is not in A.

if you write ( $\phi$, {a, b}) then $\phi \subseteq \{a\} \subseteq \{a, b\} $ or $ \phi \subseteq \{b\} \subseteq \{a, b\} $ which is violating the definition of covering relation.

So, for every pair, add only one element in B which is not in A (or) make the hasse diagram and then check.

subset B covers subset A when you add only one element to subset A which is not in A.

if you write ( $\phi$, {a, b}) then $\phi \subseteq \{a\} \subseteq \{a, b\} $ or $ \phi \subseteq \{b\} \subseteq \{a, b\} $ which is violating the definition of covering relation.

So, for every pair, add only one element in B which is not in A (or) make the hasse diagram and then check.