Which statement about oxidation and reduction is correct?

Correct answer: In a redox reaction, one substance loses electrons and another gains electrons; therefore, both processes occur simultaneously.

A redox reaction involves transfer of electrons. The substance that loses electrons is oxidised, and the substance that gains electrons is reduced. Electron loss and electron gain occur simultaneously in the same reaction, so option B is correct. A colour change may sometimes indicate a reaction, but it is not the definition of oxidation or reduction.