Recall important properties and uses of helium

Helium is a noble gas. Its key relevant properties are:

• It is chemically inert.
• It is non-inflammable.
• It has very low boiling point, so it is used in cryogenics.
• It is lighter than air, but heavier than hydrogen.

Check each option

Option A

Statement: It is used to fill gas balloons instead of hydrogen because it is lighter and non–inflammable

• Helium is indeed non-inflammable, which is why it is preferred over hydrogen in balloons.
• However, helium is not lighter than hydrogen.
• In fact, hydrogen is the lightest gas, and helium is heavier than hydrogen.

Therefore, the reason given is incorrect.

So, Option A is incorrect.

Option B

Statement: It is used in gas-cooled nuclear reactors

• Helium is inert and does not become radioactive easily.
• Hence it is used as a cooling gas in some nuclear reactors.

So, Option B is correct.

Option C

Statement: It is used to produce and sustain powerful superconducting reagents

• This appears to mean superconducting magnets (common textbook phrasing).
• Liquid helium is used to maintain very low temperatures required for superconductivity.

So, Option C is correct in intent.

Option D

Statement: It is used as cryogenic agent for carrying out experiments at low temperatures

• Because of its extremely low boiling point, helium is widely used as a cryogenic agent.

So, Option D is correct.

Final conclusion

The incorrect statement is: $\boxed{\text{A}}$

Comparison with stored correct answer

Stored correct answer: A

My derived answer is also A, so they agree.