The correct answer is D) Requested.
Another word for summoned that would have the same effect in the sentence is Requested.
Summon is defined as to call upon to do something. A synonym could be requested. To request is defined as to invite, to ask for something or a petition. That is why summoned can be substituted by requested in the sentence.
“Since this country was founded, each generation of Americans has been requested to give testimony to its national loyalty”.
