Each tile is either marked with a plus sign or a minus sign to indicate "positive 1" or "negative 1" respectively.
Combining the tiles is basically adding 1 or negative 1 together.
Look at choice C and notice we have 2 sets of a group of 5 "negative 1" tiles, giving 2*5 = 5+5 = 10 of such tiles total. The final result is -10. You can think of it like "10 IOUs" or "you are 10 dollars in debt".
