H3PO3 is a good reducing agent than H3PO4 . The reducing nature of these acids is based on the no of hydrogen atoms bonded to the central metal atom. As more number of H attached to phosphorous more is its reducing nature. In the case of H3PO3, there is one P−H bond which is responsible for it's reducing character whereas there is no such bonding in case of H3PO4.
Hence option A is correct.