Explanation

RuBP carboxylase/ oxygenase (RUBISCO) is an enzyme present in the stroma of chloroplast. This enzyme is responsible for primary carboxylation in $C_{3}$-plants as a 5C sugar RuBP acts as carbon dioxide acceptor in the presence of this enzyme and produces 6C unstable compound which then splits into two molecules of 3-phosphoglyceric acid (3C compound), the first stable product of $C_{3}$-cycle.