Photorespiration reaction catalyzed by RuBisCo is shown below :...

In photorespiration, the enzyme RuBisCo acts as an oxygenase.

It catalyzes the oxygenation of Ribulose-1,5-bisphosphate (RuBP) to form one molecule of a 3-carbon compound, 3-phosphoglycerate, and one molecule of a 2-carbon compound, 2-phosphoglycolate.

The reaction is:
RuBP $+$ O$_2$ $\rightarrow$ 3-Phosphoglycerate $+$ 2-Phosphoglycolate

Thus, the product X is 2-Phosphoglycolate.

Answer: 2-Phosphoglycolate