It is high time they ______ (improve) their communication skills.

• It is time/ It is high time:
- It is time/ It is high time থাকলে আর এর পর subject বসলে subject এরপরের verb টি past form হয়।
- Example: It is high time we started the meeting.

• It is time ও It is high time এর পরে subject না এসে, কোন verb আসলে ঐ verb এর Infinitive form হবে।
- Example: It is high time to change our attitude.

• বাক্যে It is time এর পর subject (they) আছে তাই সঠিক উত্তর - improved.

• Correct sentence: It is high time they improved their communication skills.

Source: A Passage To The English Language: S. M. Zakir Hussain.