In basic electricity, which of the following sources directly produce alternating current (AC) as their usual output under normal operation?

Introduction / Context:
Alternating current, commonly called AC, is the type of electrical current used in most household and industrial power systems. It periodically reverses direction, unlike direct current, or DC, which flows in one direction only. Different power sources generate different types of current. Understanding which sources produce AC directly is important for basic electrical engineering, power distribution, and practical applications. This question asks you to identify which listed source naturally produces AC as its main output.

Given Data / Assumptions:
• The options include solar power plants, hydro electric generators, and batteries. • The question focuses on the type of current produced directly by these sources. • We assume standard designs of commercial photovoltaic systems, hydro turbines, and chemical batteries. • AC means current that reverses direction periodically, while DC flows in one direction.

Concept / Approach:
Hydro electric generators are rotating machines that operate on electromagnetic induction. As the generator rotor spins, it cuts magnetic field lines and induces an alternating voltage in the stator windings, producing AC directly. Batteries, by contrast, are electrochemical devices that maintain a steady potential difference between their terminals and deliver direct current. Solar photovoltaic panels also produce DC, not AC; their output is typically fed into inverters that convert DC to AC for grid use. Therefore, among the listed sources, only hydro electric generators produce alternating current directly as their primary electrical output.

Step-by-Step Solution:
Step 1: Recall that AC generators, also called alternators, are designed to produce alternating current through rotating coils and magnetic fields. Step 2: Recognise that hydro electric power plants use turbines to rotate large alternators. Step 3: Understand that the voltage waveform from these alternators is sinusoidal and alternates in polarity, creating AC. Step 4: Consider batteries, which provide a fixed polarity and deliver DC current under load. Step 5: Consider photovoltaic solar panels, which generate DC at their terminals and require inverters for AC output. Step 6: Conclude that hydro electric generators are the only option here that directly produce AC.

Verification / Alternative check:
Power system diagrams show that large hydro electric plants connect alternators directly to the AC transmission network. Their terminal voltages are synchronised with the grid frequency, such as 50 or 60 hertz. In contrast, data sheets for solar panels list direct current output characteristics, and solar farms include grid tied inverters to convert that DC into AC suitable for distribution. Battery specifications likewise describe DC voltage ratings and capacities, with separate devices called inverters or DC to AC converters needed when AC output is desired. These standard practices confirm that only hydro electric generators among the listed options naturally output AC.

Why Other Options Are Wrong:
Option A, solar power plants, are based on photovoltaic cells that generate DC electricity at the panel level, even though the final delivered power to the grid is AC after inversion. Option C, batteries, supply DC for devices such as phones, torches, and vehicles; they do not internally generate alternating current. Option D, all of the above, is incorrect because it ignores the crucial distinction between direct and alternating current generation and the role of inverters in solar and battery systems. Thus, only hydro electric generators directly produce AC.

Common Pitfalls:
A common misunderstanding is to think that if a power plant ultimately supplies AC to homes, it must generate AC directly. In reality, some sources such as solar and battery storage rely on power electronics to convert DC to AC before grid connection. Another pitfall is to overlook how generators work and to treat all electrical sources as equivalent. Remembering that rotating machines with coils in changing magnetic fields are the classic AC generators will help you quickly identify which sources produce alternating current directly.

Final Answer:
The correct choice is Hydro electric generators driven by flowing water, because these rotating machines produce alternating current directly through electromagnetic induction, while solar panels and batteries inherently generate direct current.