In table 1.1, the highest opportunity cost of missiles in terms of houses is:

Opportunity cost basically means that you have a finite amount of resources, whether it's raw materials, labor, capital, whatever... and with those resources you can choose what combination of things you want to produce. The more you make of one thing, the less you can make of another. The opportunity cost of increasing production of houses from 17 to 21 is one missile. The highest opportunity cost of houses is 4 missiles or 0 missile.