Here's why:
* Caste is a complex system: It's not a simple, clear-cut classification. There are many sub-castes, and a person's caste can be influenced by various factors.
* Surnames are not always indicative of caste: Many surnames in Bengal are adopted or changed over time, making them unreliable indicators of caste.
* Caste is often seen as a sensitive topic: Asking about someone's caste can be perceived as intrusive and inappropriate.
Therefore, it's not possible to determine the Bengali caste associated with the surname Biswas. It's important to approach these matters with sensitivity and respect.