You must work hard for success. Make it compound -

• Compound sentence :
- Compound sentence-এ একের অধিক principal clause থাকে যাদেরকে co-ordinate clause বলা হয়।
- অর্থাৎ Compound sentence এ দুই বা ততোধিক principal clause বা co-ordinate clause থাকে।
- এছাড়া Compound sentence এ সাধারণত and, or, but, yet, so, therefore, otherwise, else, both --- and , either --- or , neither — nor, not only --- but also ইত্যাদি co-ordinating conjunction দ্বারা দুইটি principal clause যুক্ত থাকে।

অপশন 'খ' এর বাক্যেটিতে (Work hard and you will succeed.) conjunction ‘and’ রয়েছে, তাই এটিই সঠিক উত্তর। 

• Example of Compound sentence :
- Work hard and you can prosper in life.
- Do or die.
- I went to the store, and I bought some groceries."

Source: Advanced Learner's by Chowdhury & Hossain, A Passage to the English Language, S.M. Zakir Hussain.