Let us concentrate on improving the education system in our country.
Here, the underlined word is-

• Let us concentrate on improving the education system in our country.
- Here, the underlined word is- খ) Gerund
- বাক্যটিতে preposition 'on' -এর object হিসেবে 'improving' -word টি হলো Gerund.

• Gerund
- Verb -এর সাথে ing যোগ হয়ে যদি noun -এর কাজ করে অর্থাৎ, একই সাথে Verb ও noun -এর কাজ করে, তখন তাকে Gerund বলে।
- সহজে → Gerund = Verb + ing = noun = Verb + noun -এর কাজ করে।
- Gerunds don’t describe action—they act as nouns.

• একটি বাক্যে Gerund বিভিন্ন অবস্থানে বসতে পারে।
- বাক্যে verb+ ing যুক্ত word টি যদি বাক্যের preposition -এর পরে object হিসেবে বসে, তবে তা সাধারণত Gerund হবে।

• More examples:
1. I am tired of waiting.
2. He is fond of swimming.

• Functions of the Gerund:
1. As a subject of a verb: Rising early is a good habit.
2. As an object of a verb: I like reading poetry.
3. As an object of a preposition: I am tired of waiting.
4. As a complement of a verb: Seeing is believing.
5. As absolutely (part of a compound noun): This is my writing table.

Source:
1. A passage to the English Language by S M Zakir Hussain.
2. High School English Grammar and Composition by Wren And Martin.