The truth is that he has deceived us. Here 'that he has deceived us" is a/an -

• The truth is that he has deceived us. Here the underlined part is a - Noun clause.

• A noun clause is a group of words that band together and act like a noun. Nouns clauses are used when a single word isn't enough. They're always dependent clauses.

• একটি বাক্যের চারটি স্থানে Noun clause  বসতে পারে -
1. subject হিসাবে 
2. transitive verb এর object হিসেবে
3. Linking verb এরপরে complement হিসাবে 
4. Preposition এরপরে

- প্রদত্ত বাক্যে Linking verb (is) এরপরে complement হিসাবে  that he has deceived us বসেছে বিধায় এটি একটি noun clause.