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I honestly hadn't heard of an element with a symbol J so I did some research and found that the Iodine (I) was once called Jodium (J) so I'm going to assume that J in this problem is also Iodine (I).

Ionic bond occurs between ions with opposite charges + and - or between metals and nonmetals.

So with this concept in mind, let us find which among the choices suit the definition of an ionic bond.

a. L and N - No such thing as element L in the periodic table.

b. K and N - K or Potassium is a metal while Nitrogen is a non-metal so the form of bond that will exist between them is ionic bond.

c. J and M - No such a thing as stand-alone M element.

d. J and J - Iodine and Iodine. Iodine is non-metal. Bonding between non-metals is covalent bond, not ionic.