Tania, accompanied by her brother _____ at the party.

Was

Were

Are

Tania, accompanied by her brother, was at the party.

تانیہ ، اس کے ساتھ اپنے بھائی کے ساتھ ، پارٹی میں تھیں۔

The phrase "accompanied by her brother" does not change the singular subject "Tania".

So the singular verb "was" is correct.