Which of the following options should be used to replace the underlined portion to correct the sentence?
We didn't expect so many people to turn on for the event, and we had to rearrange the seating.

Turn up (phrasal verb):
Meaning - (ক) উপস্থিত হওয়া; আসা: I doubt whether he will turn up at the meeting.
(খ) বিশেষত দৈবক্রমে পাওয়া যাওয়া: The stolen umbrella turned up in a flea market.
(গ) (সুযোগ ইত্যাদি) ঘটা: He’s hopeful that something may yet turn up.

• তাই অর্থ বিবেচনায় Correct Sentence - We didn't expect so many people to turn up for the event, and we had to rearrange the seating.

Other options,
Turn in - শুতে যাওয়া।
Turn out - প্রমাণিত হওয়া; শেষ পর্যন্ত হওয়া।
Turn (somebody/something) over - উলটানো; উলটে যাওয়া।

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