The expressions 100×10 and the more complicated
(100 - 4a)(10 + a)
are calculating annual (expected) revenue. In 100×10 the 100 is the number of decks and the 10 is the price per deck. In (100 - 4a)(10 + a) the 100-4a is the number of decks and the 10+a is the price per deck. So a must be the amount the price increases.
Answer: amount of price increase, last choice
