The reducing agent in a material is the one that a. gains electrons. b. increases its atomic number. c. is reduced. d. is oxidized.

Answer is: D. is oxidized.

Reducing agent is an element  or compound that loses an electron to another chemical species in a redox chemical reaction and they have been oxidized.

For example, chemical equation: Ca + H₃PO₄ → Ca₃(PO₄)₂ + H₂.

Oxidation reaction: Ca⁰ → Ca⁺² + 2e⁻ /×3.

3Ca⁰ → 3Ca⁺² + 6e⁻

Reduction reaction: 6e⁻ + 2H₃⁺¹ → 3H₂⁰.

Balanced chemical equation: 3Ca + 2H₃PO₄ → Ca₃(PO₄)₂ + 3H₂.

Calcium is stronger reducing agent than hydrogen, gives electrons easier.

Calcium is oxidized, it changes oxidation number from 0 to +2.