The alternating current cannot be measured by a dc ammeter, because

The correct answer is Option (4) → The average value of alternating current for a complete cycle is zero

A dc ammeter (moving-coil type) has a deflection proportional to the current and reversing with it, so it responds to the average current. An alternating current reverses every half cycle, and over a complete cycle its average value is zero.

    \( \displaystyle I_{\mathrm{avg}} = \frac{1}{T}\int_{0}^{T} I_0\sin\omega t\,dt = 0 \)

The pointer cannot follow the rapid reversals, so it stays at zero. Alternating currents are measured with meters based on the heating effect, which depends on \( I^{2} \) and gives the rms value.

NCERT (2026-27):
AC Voltage Applied to a Resistor – “… We see that, like the applied voltage, the current varies sinusoidally and has corresponding positive and negative values during each cycle. Thus, the sum of the instantaneous current values over one complete cycle is zero, and the average current is zero.”