Assertion: 1 mole of oxygen and nitrogen gas have the same volume at same temperature and pressure.
Reason: 1 mole gas at NTP occupies 22.4L volume at STP.
(A)- Both assertion and reason are correct and reason is the correct explanation for assertion.
(B)- Both assertion and reason are correct but reason is not the correct explanation for assertion.
(C)- Assertion is correct but reason is incorrect.
(D)- Both assertion and reason are incorrect.

Hint: STP refers to standard temperature and pressure. Volume of 1 mole of a gas is called molar volume. Molar volume tells us about the number density of the gas. Molar volumes of gases at STP are the same.

Complete step by step solution:
Let’s look at the given question:
- The assertion in the question states that 1 mole oxygen and nitrogen have the same volume at same temperature and pressure. This statement is correct in accordance with Avogadro's law.
- The Avogadro’s law of gaseous volumes states that at NTP one mole of any gas occupies 22.4L volume at same temperature and pressure.
Now, let us see what reason is given for the assertion.
- The reason states that 1 mole gas at NTP occupies 22.4L volume at STP. This is also correct in accordance with Avogadro's law of gaseous volumes. It is actually the statement of the law.
Since, the reason correctly explains the statement in the assertion.
Therefore, both assertion and reason are correct and reason is the correct explanation of assertion.

Hence, the answer for the given question is option (A).

Note: The standard temperature is 273.15K and standard pressure is 1 atm for a gas. All gases have the same molar volume at STP because they have the same number density. The number density of a gas is the number of particles that are present in a particular volume.