The ‘blue moon’ phenomenon occurs when

The correct answer is (a). A blue moon is the second full moon in a calendar month. It occurs about every 2.7 years. The term “blue moon” is also used to refer to any rare or unusual event.

Option (b) is incorrect. Four full moons can appear in two consecutive months of the same calendar year, but this is not considered a blue moon.

Option (c) is incorrect. Two full moons cannot appear in the same month thrice in a calendar year.

Option (d) is incorrect. The blue moon phenomenon is a real astronomical event, not a myth.