What is 'Blue Moon'?

The correct answer is Option (3) → Second full moon of the two full moons in a month

Full moons come about every 29.5 days, a little less than most calendar months. So now and then a month of 30 or 31 days has two full moons, and the second full moon in the same calendar month is called a Blue Moon. It happens only about once in two to three years, which is where the saying “once in a blue moon” comes from; the Moon does not actually look blue.