On what basis will an extremely small but brightest focal point become effective?

Correct answer: On the combined effect of colour intensity, isolation, and surrounding context

Focal strength is determined by relationships, not physical size alone. A very small area can dominate when its colour is highly intense, it is isolated from competing elements, and the surrounding context makes its brightness noticeable. These cues reinforce one another and create emphasis. Symmetry or background texture by itself is insufficient, and small size alone would normally reduce attention. Thus option A gives the complete and correct basis.