Which basic principle of individual choice do these statements best illustrate? the cost of something is what you give up to get it markets are usually a good way to organize economic activity people usually exploit opportunities to make themselves better off. trade can make everyone better off?

I think the principle that under the market economy, people have to be ready to take advantage of employment and opportunities when the price of commodities like metals is up and be prepared to work extra hard at those times to save up for lean times when there is a recession or depression because the market economy is cyclical - boom or bust. The economic principle involved in these examples is that people take advantage of opportunities that come up involving things to purchase say during sales and when trade brings cheaper but still good items into the local economy.