The correct option is
A Are / they are
The correct answer is Option A.
The word 'ARE' is used when we refer to two or more things. In the given sentence, there is a reference to two people- Jack and Benny.
Thus, ARE is the correct word that completes the sentence.
'THEY ARE' completes the second sentence because THEY is plural and there is a reference to two people in the previous sentence.
The remaining options are incorrect because:
Option B: AREN'T is a negation and there is no such meaning in the sentence. THERE ARE is not the appropriate pronoun to refer to two people.
Option C: AM is used only with I. THEIR is used to show possession.
Option D: IS is used to refer to singular words. In the given sentence we are referring to two people.
Hence, Option A is the correct answer.