They are used _________ customer complaints, so they remain calm and professional in difficult situations.

• সঠিক উত্তরটি হবে - to handling.
- Complete sentence: They are used to handling customer complaints, so they remain calm and professional in difficult situations.

• Used to (phrase)
English Meaning: shows that a particular thing always happened or was true in the past, especially if it no longer happens or is no longer true.
Bangla Meaning: অতীতের নিত্য বা প্রায়িক ঘটনা নির্দেশে ব্যবহৃত।

• Modal auxiliary এর নিয়মানুযায়ী,
- শুধু Used to এর পর Verb এর base form বসে।
যেমন:
- I used to eat meat, but now I’m a vegetarian.
- He used to go to our school.

• তবে be (am, is, are, was, were) + used to থাকলে এর পর Verb+ing ব্যবহৃত হয়।
• Be used to/get used to এর পরে noun অথবা Gerund বসে।
• Structure: subject + be/get + used to + [verb + ing]/noun.
যেমন:
- I'm used to living in Paris.
- She’s used to working hard.
- He was used to waking up early for his morning jog.
- I am used to studying in crowded places.

Source:
1. Cambridge Dictionary.
2. Accessible Dictionary by Bangla Academy.