Select the most appropriate set of idioms to fill in the blanks.
A new project is ________ in Central Delhi. My father, being the chief architect, is always _________, even on weekends.

Option D: Correct.

Understanding the meaning of idioms and phrases is the key to this question. Also, identifying, if the idiom is correct is important.

On the air- broadcasting

On the cards- a future event likely to happen

On call- being available usually outside the designated working time

On the boil- on the verge or at peak

So, option D fits best, as the sentence tries to convey that a new project in Central Delhi is being worked upon and the speaker's father is expected to be available even on the weekends.