A buffer that is most effective at a pH of about 4.5 is;

Ans. is 'a' i.e. Acetate BufferRef Guyton, 10/e, p349A buffer system is most effective when the pH of the system is near its pka valueLet's see the pka of the buffers given in the questionBufferPkaPhosphateBicarbonateAcetateTris 6.8 6.14.76 (Harper 25th/e p24) 8.3 (Harper 25th/ep23)* It's the acetate buffer which has its pka (4.76) value nearest to the pH of the system (4.5), hence it will be most effective at this pH.