Assertion: Separation of isotopes is possible because of the difference in electron number of isotopes.
Reason: Isotope of an element can be separated by using a mass spectrometer.
A. If both assertion and reason are true and the reason is the correct explanation of the assertion.
B.If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If the assertion is true but the reason is false.
D.If the assertion and reason both are false.
E. If the assertion is false but the reason is true.

Hint: A mass Spectrometer determines the mass of a molecule by measuring the mass to charge ratio of ions of that particular molecule. It operates on the principle that moving ions may be deflected by magnetic and electric fields. Isotopes are nothing but atoms with the same number of protons but different number of neutrons.

Complete step by step solution:
Mass spectrometer is a laboratory device that measures the mass-to-charge ratio ($\dfrac{m}{z}$) of ionised molecules. The device is used to identify the molecules and their elemental composition. The mass spectrometer is a common tool in chemistry, biology, and physics. It is used to identify the molecules in a sample, to measure their masses, and to determine their elemental compositions. The mass spectrometer consists of three essential parts: the ion source, the mass analyzer, and the detector.

Sample ionisation occurs at the ion source. The ionised molecules are then passed through the mass analyzer, which separates the ions according to their mass-to-charge ratios. The detector then determines how many ions are present at each mass-to-charge ratio.. The mass spectrometer can be used to identify molecules by their mass, to measure the masses of molecules, and to determine the elemental compositions of molecules.

The atoms of the same element that have the same number of protons but different mass numbers, which means the number of neutrons are different, are called isotopes. For example, carbon has six protons, but can have $12$, $13$, or $14$ mass numbers. These differing forms of carbon are called isotopes. Now isotopes of any element have different mass to charge ratio, and mass spectrometers separate the isotopes of different elements based on their mass to electrical charge ratio ($\dfrac{m}{z}$). Hence from the above discussion we see that assertion is wrong but reason is correct.

So, option E is correct.

Note: We should know one more important piece of information that mass spectrometry is called spectrometry but not mass spectroscopy. The main reason is that it is an analytical technique where molecules are converted into their ions by a fragmentation process therefore no interaction of the electromagnetic radiation is used here.