The question asks for the number of chiral centres in [B], but the structure of compound [B] is not provided in the prompt.

To determine the number of chiral centres, I must inspect the actual molecular structure and identify all tetrahedral atoms (usually carbon) attached to four different groups.

Why the answer cannot be derived from the given information

1. A chiral centre depends entirely on the specific connectivity and substituents in the molecule.
2. Since compound [B] is missing, there is no way to count its stereogenic centres reliably.
3. Therefore, the answer cannot be independently verified from the prompt alone.

Comparison with stored answer

The stored correct answer is 4, but without the structure of [B], I cannot confirm whether this is correct.

Please provide the reaction scheme or the structure/image of compound [B], and I can then count the chiral centres step-by-step.