none of the answers to the first sentence are correct, you could use "would have been AT the party" though, the second sentence is C (by)

Hello! #1: "Would have been" is correct. "Would have" is a modal verb indicating something that was going to happen voluntarily, but didn't happen. #2: "by" is correct as this is a preposition indicating the extent of something - in this case the extent or degree of the increase.

1) "he must have been ill, else he would have been at the party last night" (OR also works, however else works best in my opinion) 'at the party' instead of 'to the party'. 2) The price of the computers has increased by 10% recently (if you are referring to rise in sales) OR the price of the computers has increased to 10% recently (this would imply the computers were once selling for a price lower then 10% but has now increased in price to 10%). But I believe the answer you are looking for in this situation is BY