He understandswhy she made that decision. Here the underlined part is a/an -

• He understands why she made that decision. Here the underlined part is a - Noun clause.

• 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 এরপর complement হিসাবে 
4. Preposition এর Object হিসেবে।

• প্রদত্ত বাক্যে 'why she made that decision' clause টি transitive verb এর object হিসাবে এটি noun clause.