Shecalled offthe meeting due to unforeseen circumstances. Here the underlined part is a/an -

• She called off the meeting due to unforeseen circumstances. Here the underlined part is a/an - Group verb.

• Phrasal Verb/ Group verb (গুচ্ছ ক্রিয়া): 
- বাক্যে Verb + preposition/adverb থাকলে সেটা Phrasal Verb/ Group verb হয়।
- যেমন - look down upon, give up, look after.
- তাই এখানে called off (verb + preposition) হচ্ছে phrasal verb/ group verb.
- যেহেতু অপশনে group verb আছে, তাই এটিই উত্তর।