A) U.S. soldiers were caught smuggling guns and butter into North Vietnam.
B) Mexican Americans living along the U.S.-Mexico border protested against harassment by U.S. immigration authorities.
C) Cuban exiles were caught trying to re-enter Cuba.
D) North Koreans captured a U.S. spy ship off the coast of North Korea.
The only one of these answers that is anywhere near to being correct is D. The USS Pueblo was an American spy ship that was captured by the North Korean navy.
The Pueblo was on a mission to collect electronic intelligence about North Korea. This is intelligence about North Korean radar, radio transmissions, and things like that. While the Pueblo was doing that, the North Koreans boarded and captured it.
This was an embarassing incident for the US. The sailors were held by the North Koreans for almost a year. North Korea used them and the incident for propaganda purposes. The US had to apologize and promise to stop spying before North Korea returned the sailors (though the US retracted the apology and the promise after they were released).