The correct answer is - খ) leave
Complete sentence: You had better leave now.

Had better, had rather, had sooner, would rather/would sooner, rather than/sooner, as soon ইত্যাদির পরে infinitive (to) বসে না।
- You had better start at once (to start নয়)

• Infinitive
- Verb এর present form এর আগে to বসিয়ে Infinitive গঠন করা হয়।
- অর্থাৎ, Infinitive = to + verb.

Infintive দুই রকম হতে পারে-
1. To-যুক্ত infinitive এবং
2. To-বিহীন infinitive বা Bare Infinitive.
Infinitive form বিভিন্ন ভাবে ব্যবহৃত হতে পারে।
যেমন: Subject of verb, Object of verb, Complement of verb, object of preposition, etc.

• Bare Infinitive
নিম্নলিখিত verb-এর পরে infinitive (to) বসে না।
(a) can, do, may, must, shall, will, would, have, should এর পরে -
- I may help you.
- He must do it.

(b) need এবং dare - auxiliary verb হিসেবে ব্যবহৃত হলে:
- I dared not wake him.
- They need not do anything.
কিন্তু affirmative বা হ্যাঁ বোধক হলে need ও dare এর পরে to বসে।
- I need to buy a pen.

(c) please, see, know, hear, bid, feel, hear, let, see, make, watch-এর পরে -
- I saw him go.
- The old man watched his companions fall down dead.

কিন্তু Passive Voice-এ see, hear, behold, watch এবং make এর পরে full infinitive ব্যবহৃত হয়।
- Active: I saw him go.
- Passive: He was seen to go.

(d) Had better, had rather, had sooner, would rather/would sooner, rather than/sooner, as soon ইত্যাদির পরে -
- You had better start at once (to start নয়)

(e) "Better" adjective-এর পরে to উহ্য থাকে।
- Better stay here now.

Source:
A passage to the English Language by S M Zakir Hussain.