In the following sentence, an idiom is highlighted. Choose the most suitable meaning of the expression "on the grapevine" as used in the sentence. Sentence: I heard it on the grapevine that the vacancies are going to be increased this year for many posts.

Introduction / Context:
This question tests your knowledge of the idiom "on the grapevine". The sentence describes hearing about an increase in vacancies and hints at the source of this information. You must choose the option that best reflects the idiomatic meaning of "on the grapevine" in modern English.

Given Data / Assumptions:

• Idiom: "on the grapevine".
• Sentence context: hearing about possible changes in vacancies for many posts.
• Option A: To hear some official news about something.
• Option B: To inform people about something.
• Option C: To acquire information by unofficial communication.
• Option D: To be sure of something.

Concept / Approach:
"To hear something on the grapevine" means to learn it through unofficial channels, gossip, rumours or informal talk, not through formal announcements. The expression suggests a network of informal communication, similar to how grapevines spread. It does not indicate that the news is official or certain, and it does not mean that you were the one informing others.

Step-by-Step Solution:
Step 1: Note that the speaker says "I heard it on the grapevine", which points to the source of information, not the act of spreading it.Step 2: Understand that this phrase is commonly used when talking about rumours or news shared among people informally.Step 3: Examine option C, "To acquire information by unofficial communication". This exactly captures the idea of learning something through informal talk or gossip.Step 4: Examine option A, which mentions "official news". That is the opposite of what the idiom suggests.Step 5: Examine options B and D. Option B describes telling others, not hearing something. Option D, "to be sure of something", is about certainty and does not relate to the mode of communication.

Verification / Alternative check:
Consider other sentences: "I heard on the grapevine that they are getting married" or "Rumours on the grapevine suggest a merger." In both examples, the phrase clearly refers to informal or unofficial information, often not confirmed. Substituting "official news" or "being sure" would distort the meaning.

Why Other Options Are Wrong:
Option A incorrectly attaches the idiom to official sources, which contradicts the typical use. Option B changes the role from listener to speaker. Option D introduces the idea of certainty, whereas information gained on the grapevine is often uncertain or unverified.

Common Pitfalls:
Some learners may confuse "grapevine" with an official news network or think of it simply as any information channel. However, the idiom always carries the nuance of informality and rumour. Remembering this nuance helps in questions where subtle differences in options are tested.

Final Answer:
The idiom "on the grapevine" means to acquire information by unofficial communication, so option C is correct.