They have become ___________ swimming in the cold ocean by practicing every day.

• শূন্যস্থানে সঠিক উত্তর হবে - habituated to.
- Complete sentence: They have become habituated to swimming in the cold ocean by practicing every day.
- Bangla Meaning: প্রতিদিন অনুশীলনের মাধ্যমে তারা ঠান্ডা সাগরে সাঁতার কাটতে অভ্যস্ত হয়ে উঠেছে।

• Habituate somebody/oneself to something:
English Meaning: used to something, especially something unpleasant।
Bangla Meaning: (আনুষ্ঠানিক) (কাউকে/নিজেকে কোনোকিছুতে) অভ্যস্ত করানো।

• এছাড়াও,  কোনো বাক্যে mind, cannot help, could not help, with a view to, look forward to, be used to, worth, accustomed to, habituated to, ইত্যাদি শব্দ সমূহের পর কোনো verb বসলে সেই verb এর সাথে ing যুক্ত করতে হয়।
- অর্থাৎ habituated to + Verb+ ing.

• Example Sentences:
- We find children's emotional needs difficult to respond to because we are habitu।ated to disregarding our own.
- As a frequent traveler, she became habituated to adapting quickly to different time zones.

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