He received a promotion while he _______ on a challenging project.

• While দ্বারা দুইটি বাক্যাংশ যুক্ত হলে,
- While এর পূর্বে অংশ টি past indefinite এ হলে,
- এরপরের অংশে past continuous tense হয়। 
Complete Sentence: He received a promotion while he was working on a challenging project.

এখানে, while এর পূর্বের অংশটি past indefinite (He received a promotion) এ আছে তাই পরের অংশে হবে past continous tense ( he was working on a challenging project)।
Example:
- The phone rang while I was cooking the dinner.
- My sister arrived while I was reading the book.

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