What kind of transition should appear between colours during blending?

Correct answer: Smooth and gradual

Blending is governed by the principle of gradual tonal or colour change. The edge of one wet or softly applied colour should merge progressively into the next, without a sharp dividing line; therefore option C is correct. A sudden, hard transition leaves a visible boundary, a broken transition interrupts continuity, and black alone is not a blending method or required result.