Which one is correct about ferrites?
(a) These possess formula $A{{B}_{2}}{{O}_{4}}$(where A is divalent and B is trivalent cation)
(b) These possess spinel structure.
(c) $MgA{{l}_{2}}{{O}_{4}}$ is a ferrite.
(d) All of the above

Hint: As we know that ferrites are named to the class of the ferromagnetic compounds with the general formula $A{{B}_{2}}{{O}_{4}}$ where A is a divalent cation and B is a trivalent cation. These are one of the most essential materials in the electronic industry and can be generally classified into soft and hard ferrites. So we will attempt these questions by checking whether the statement provided matches the properties of spinel or not.

Complete answer:
First let us understand about ferrites:-
-Ferrites are a large class of oxides containing a trivalent and a divalent cation of same or different metal that have been investigated and applied to a wide variety of fields ranging from old technologies like circuitry, magnetic recording, permanent magnets and microwave devices to the most recent developments in areas like bioimaging and gas sensing.
Now let us check the statements provided in the question:-
(a) These possess formula $A{{B}_{2}}{{O}_{4}}$(where A is divalent and B is trivalent cation):-
Ferrites are the compounds with general formula $A{{B}_{2}}{{O}_{4}}$where A is a divalent cation and B is a trivalent cation. In the structures of ferrites oxide ion occupies the ccp sites, divalent cations occupies one-eighth of the tetrahedral voids and trivalent cations occupies half of the octahedral voids. Therefore, this statement is true.
(b) These possess spinel structure:-
Ferrites do have spinel structure therefore this statement is also true.
(c) $MgA{{l}_{2}}{{O}_{4}}$ is a ferrite:-
$MgA{{l}_{2}}{{O}_{4}}$is a ferrite with a spinel structure where Mg is a divalent cation and Al is a trivalent cation. Hence this statement is also true.

From the above data we conclude that, the correct answer is (d) All of the above

Note:
Few more properties which are helpful in the questions related to ferrites are as follows:-
-Material formed by ferrite compounds are used as electromagnetic wave absorbers because ferrite absorbers have a wide frequency bandwidth in very high and ultrahigh frequency bands.
-These compounds are basically ferromagnetic oxide materials which possess high resistivity and permeability.