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# The energy in stars is because of -A. RadioactivityB. Nuclear fissionC. Nuclear fusionD. Photoelectric phenomenon

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Hint: - The main source of energy in stars is the carbon cycle (also called the CNO cycle for carbon, nitrogen, and oxygen), in which hydrogen is transformed into helium.
- Fusion is the process where two hydrogen atoms combine to form a helium atom, releasing huge energy.

Complete step by step solution:
The most basic property of stars is that their radiant energy must derive from internal sources. Given the great length of time that stars endure (some 10 billion years in the case of the Sun), it can be shown that neither chemical nor gravitational effects could yield the required energies. Instead, the cause must be nuclear events wherein lighter nuclei are fused to create heavier nuclei.

For the Sun and other normal main-sequence stars, the source of energy lies in the conversion of hydrogen to helium.
The fusion reaction is a very efficient process, releasing a huge amount of energy. This is because a single helium atom contains less mass than two hydrogen atoms. The excess mass is released as energy.

So, Nuclear fusion is responsible for the energy in stars.

The correct option is (C).

Note: - How much energy the fusion reaction releases, we can calculate that using the famous formula by Albert Einstein $E = M{c^2}$ - Nuclear fusion is a very difficult process to control.
- When the new star reaches a certain size nuclear fusion starts.