Let’s analyze each statement in the context of the classical (and standard) Lassaigne’s test:

Statement (I)

“Nitrogen, sulphur, halogen, and phosphorus present in an organic compound are detected by Lassaigne's Test.”

This is true.

The Lassaigne’s test is used to detect N, S, X (halogens), and can also be adapted for P in organic compounds.

Statement (II)

“The elements present in the compound are converted from covalent form into ionic form by fusing the compound with Magnesium in Lassaigne's test.”

This is false, because:

In the classical Lassaigne’s test, one fuses the organic compound with sodium metal (Na), not magnesium.

The purpose is indeed to convert elements from their covalent forms (e.g., C–N, C–S, C–X bonds) into their ionic forms (e.g., NaCN, Na₂S, NaX, etc.). But the metal used is sodium, not magnesium.

Hence, while the overall description of “converting covalent to ionic form” is correct, the statement incorrectly specifies “magnesium” instead of “sodium,” making it false as written.

Conclusion

Statement (I) is True.

Statement (II) is False.

Therefore, the correct choice is:

(B) Statement I is true but Statement II is false.