Fill in the blank with the correct idiom from the options given below…

The correct answer is Option (3) → over the moon

To be over the moon means to be extremely happy and delighted. Getting a much coveted (greatly desired) job is a reason for great joy, so Ravi is over the moon.
Example: She was over the moon when she heard she had won the scholarship.

Why the Other Options are Incorrect?

Option 1: on the fence — Incorrect. On the fence means unable to decide between two choices; Ravi has already got the job, so there is nothing to decide.
Example: I am still on the fence about which college to join.

Option 2: under the weather — Incorrect. Under the weather means slightly ill; it does not fit the good news.
Example: He stayed home because he was feeling under the weather.

Option 4: barking up the wrong tree — Incorrect. Barking up the wrong tree means looking in the wrong place or blaming the wrong person; it has nothing to do with getting a job.
Example: If you think I broke the vase, you are barking up the wrong tree.