Which of the Following Transactions Would Take Place in the 'Underground

Which of the following transactions would take place in the 'underground economy'?

A) Elizabeth tells her mother she's going to work but really goes to see a movie.
B) Paul buys 15 litres of petrol for $1.45 per litre, not realising that 40 cents per litre of that price goes to the government as tax revenue.
C) John makes chain-link belts and necklaces and sells them at a local market for cash to avoid paying taxes.
D) Matt bought an iPhone for $400 but decided to sell it on eBay instead of keeping it.