Hot-wire ammeters are used to measure

Correct option is D

The correct answer is (d) both AC and DC

Explanation:
• A hot-wire ammeter strictly relies on the heating effect of electric current passing through a fine resistance wire (typically platinum-iridium).
• Because the heat generated ($H = I^2Rt$) is heavily dependent on the square of the current, the direction of current flow is irrelevant.
• Thus, it can successfully measure the RMS (Root Mean Square) value of alternating current (AC) as well as direct current (DC).

Information Booster:
• One minor drawback of hot-wire instruments is that their internal scales are non-uniform (cramped at the lower end) due to the square-law response.
• Moving-coil galvanometers, by contrast, measure average values and strictly work only for DC unless coupled with a rectifier.

Additional Knowledge:
only DC (Option a)
• Moving-iron and hot-wire instruments are not restricted to DC; only permanent magnet moving coil (PMMC) instruments are DC-only.
only AC (Option b)
• While frequently utilized to read AC RMS values, they fundamentally work perfectly well with DC currents.
neither AC nor DC (Option c)
• Incorrect, as these are functional ammeters designed explicitly to gauge current flow.
None of the above/More than one of the above (Option e)
• Incorrect, as option (d) is factually accurate.

So the correct answer is (d)