Short Answer: 2000me (choice 2). (me stands for mass of electron, as compared to mp which stands for mass of proton)
Long Answer: Imagine two bean bags. Each have equal number of beans in them at the moment. Let us assume, every bean weighs 1 unit of mass. Now, take off 1000 beans from the first bag. At this moment, the 1st bean bag weighs a 1000 less than it's original weight. So, let us formulate this.
Bean Bag A: original weight = X
Bean Bag B: original weight = X, since both weigh the same in the beginning.
Now after removing 1000 beans from Bag A, it's weight becomes (X - 1000)
You are also adding those beans to Bag B, so it's weight becomes (X + 1000).
So, the difference would be
(X + 1000) - (X - 1000) = 2000 me
