In this unbalanced reaction, which atom is oxidized? hno3 + hbr → no + br2 + h2o

In redox reactions, reduction is when the oxidation state of the species reduces and oxidation is when the oxidation state of the species increases.

the balanced equation for the redox reaction is as follows;

2HNO₃ + 6HBr - - - > 2NO + 3Br₂ + 4H₂O

the oxidation state of Br in HBr is - 1

oxidation state of Br in Br₂ is 0

the oxidation state has been increased from - 1 to 0 therefore Br is being oxidised.