A reducing agent is a substance which can:
The explanation for the correct option:
(D) Reducing agent: A reducing agent is a chemical substance that can easily donate electrons to other species.
The explanation for the incorrect options:
(A) Protons do not participate in the transfer for oxidation and reduction reactions. Electrons are involved in the process.
(B) Reducing agents donate electrons rather than accepting them.
(C) Electrons are involved in the redox process rather than protons.
Hence, option (D) is correct.