Concept:
The Banking Ombudsman Scheme is a system established by the Reserve Bank of India (RBI) to resolve complaints of bank customers quickly and at low cost.
Step 1: Understand its purpose.
Customers can approach the Banking Ombudsman when they have complaints regarding banking services such as
• Delay in banking services,
• Non-payment or delay in cheque collection,
• ATM or digital banking issues,
• Deficiency in customer service.
Step 2: Analyse the options.
• Digital payment option -- Incorrect.
• Unified Payment Interface (UPI) -- Incorrect.
• Platform to pay online tax -- Incorrect.
• Grievance redressal mechanism -- Correct. \checkmark
Step 3: Final conclusion.
Hence, the Banking Ombudsman Scheme is a
\[
\boxed{\text{Grievance redressal mechanism}.}
\]