Franklin. But why Thomas?

I would say definitely Thomas, given that it was his name.

(Which makes me think of "Mr Slightly Tomas" from Absolutely, but never mind.)

Sarah. All the others are names of the children of a couple we know.

Actually, not true at all, and I see other commenters have already pointed out the 'true answer'. But I am amused at the way an infinite number of curves can pass through any arbitrary set of points, and yet the human brain 'knows' which is the correct answer, just as it 'knows' the correct answer here.