In each of the following questions on idioms and phrases, choose the alternative which best expresses the meaning of the idiom Once in a blue moon.

Introduction / Context:
This question tests understanding of a very common English idiom used in everyday conversation as well as in literature. The expression Once in a blue moon does not refer to an actual blue coloured moon, but to how rarely an event takes place. It is often used in exam passages and dialogues to describe something that hardly ever happens. Knowing this idiom is useful both for comprehension and for expressing ideas about frequency in a colourful way.

Given Data / Assumptions:

• Idiom: Once in a blue moon.
• Options: Very rarely, Common, Predictable, Ordinary.
• We must select the option that matches the established idiomatic meaning.
• We assume standard conversational English usage.

Concept / Approach:
Once in a blue moon is based on the idea that a so called blue moon, a second full moon in a calendar month, is a rare event. The idiom therefore means something that happens very rarely, almost never. The correct choice must express this sense of infrequency. Very rarely does exactly that. The other options, Common, Predictable, and Ordinary, all suggest regularity or normalness, which is the opposite of what the idiom implies. Recognising the contrast between rare and common is key to answering correctly.

Step-by-Step Solution:
Step 1: Recall the typical usage: He visits us once in a blue moon, which clearly means he hardly ever visits. Step 2: Examine option Very rarely. This phrase directly expresses extremely low frequency. Step 3: Examine option Common, which suggests something that happens often or is widespread, the opposite of rare. Step 4: Examine option Predictable, which focuses on whether something can be foreseen, not on how frequently it occurs. Step 5: Examine option Ordinary, which suggests something typical or average, again unrelated to frequency. Step 6: Observe that only Very rarely matches the meaning of Once in a blue moon. Step 7: Conclude that the correct interpretation is that the event happens very rarely.

Verification / Alternative check:
Use the idiom in different sentences: She eats sweets once in a blue moon or Opportunities like this come once in a blue moon. In both cases, replacing the idiom with very rarely preserves the meaning and tone. Replacing it with common or ordinary would completely change or even reverse the meaning. Similarly, predictable does not fit because something can be rare yet predictable, such as a scheduled event every five years. Therefore, the idiomatic sense clearly matches Very rarely.

Why Other Options Are Wrong:
Common is wrong because it suggests that something happens frequently or is found everywhere, which contrasts sharply with the idea of rarity. Predictable is wrong because it addresses whether an event can be expected or foreseen, not how often it happens. Ordinary is wrong because it describes something that is normal or average rather than rare or exceptional. None of these options conveys the key idea of infrequent occurrence that defines Once in a blue moon.

Common Pitfalls:
Because the idiom contains the word once, some learners may confuse it with phrases about single events rather than focusing on rarity. Another trap is to overanalyse the phrase as something mysterious or unusual and then choose options like ordinary or predictable simply because they look different. The safest method is to recall how native speakers use the idiom in common examples, where it clearly means almost never. Practising frequency related idioms such as once in a blue moon and every now and then helps solidify these meanings.

Final Answer:
The idiom Once in a blue moon means Very rarely.