Alternating current can not be measured by D.C. ammeter. Which of the following best describes the reason for his?

average value of A. C for complete cycle is zero

$\text{D.C. ammeter measures average current}$

$\text{For A.C., current changes direction every half cycle}$

$\text{Average over a complete cycle} = 0$

$\Rightarrow \text{No deflection in D.C. ammeter}$

The correct reason is: average value of A.C. over a complete cycle is zero.