Of those offered, flashback is the most used in this selection. The other choices have to be very carefully discussed to be eliminated, however we have to defend flashback as well.
He is thinking back to a time -- pre monster -- where all was good and all was well and he was happy with his bride. Now he thinks only of revenge. He wants only to murder the monster as the monster has done to his family. He is cataloging his current feelings with those of the past.
It is not stream of consciousness. The technique had not been developed yet. This is well organized and on one topic. Stream of consciousness doesn't work that way.
foil It is not a foil that has been created. Frankenstein hates the creation. It has robbed him of everything that is dear to him (his family mostly). A foil is not a strong enough description of the monster. He has not been created to give us incite into Frankenstein. The monster is a full blown antagonist.
Flash Forward. The device used is flash back, not flash forward. Not in this section anyway. He is looking backwards when he gets to feel the joy of having his family. He gets this joy in sleep and he dreams of what they were. They are happy people in his dreams.
Paradox A paradox is a contradiction that still is true. My favorite is "This sentence is false." If it is false, then it's true. And if it is true then the statement is false. You can't win.
It is not a paradox. Frankenstein's feelings are very straight forward.
