She watched the children ________ in the park, their laughter echoing through the air.

• Collins Dictionary এবং Longman Dictionary অনুসারে Watch এর পর verb এর base form অথবা verb+ing দুটিই হতে পারে।
- এছাড়া, কোন simple sentence এ দুটি main verb ব্যবহার করা যায় না।
- তাই একটি verb এর সাথে ing যোগ করতে হয়।

• তাই, এখানে, playing সঠিক উত্তর।
- Complete sentence: She watched the children playing in the park, their laughter echoing through the air.

• From Collins Dictionary -
• If you watch someone or something, you look at them, usually for a period of time, and pay attention to what is happening.
- The man was standing in his doorway watching him.
- He watched the waiter prepare the coffee he had ordered.
- Chris watched him sipping his brandy.
- I watched as Amy ate a few nuts.

• From Longman Dictionary -
• watch somebody/something do/doing something
- I watched him go, then went home.
- Ruth could not bear to watch her parents arguing.