NetApp Keystone is a Storage-as-a-Service (STaaS) offering that provides a flexible, pay-as-you-go subscription model. It is specifically designed to deliver a seamless hybrid cloud experience by allowing customers to consume storage on-premises, in the cloud, or in a hybrid combination, all under a single subscription. This model offers financial flexibility and operational simplicity, aligning storage costs with actual usage, which directly addresses the question's requirement for a flexible payment option across both on-premises and hybrid cloud solutions.

A. Cloud Volumes ONTAP PAYGO: This is a pay-as-you-go model, but it is exclusively for NetApp's software-defined storage solution within public cloud environments (AWS, Azure, Google Cloud), not on-premises.

B. ONTAP Select: This is a software-defined storage (SDS) product that runs on commodity hardware on-premises. It is a product, not a flexible payment or consumption model for hybrid cloud.

C. Cloud Volumes ONTAP BYOL: This "Bring Your Own License" model is for customers who have already purchased perpetual licenses for Cloud Volumes ONTAP. It applies only to public cloud deployments.

1. NetApp Keystone Solution Brief (SB-4115-0321): "NetApp Keystone is a portfolio of flexible consumption models for our data fabric products and services... available for on-premises and cloud environments, giving you the ultimate in hybrid cloud flexibility." (Page 2, "What Is NetApp Keystone?")

2. NetApp Product Documentation, "What is Cloud Volumes ONTAP?": "Cloud Volumes ONTAP is a software-only storage appliance that runs ONTAP data management software in AWS, Microsoft Azure, and Google Cloud." This document clarifies that Cloud Volumes ONTAP (both PAYGO and BYOL models) is a public cloud solution.

3. NetApp ONTAP Select Datasheet (DS-3849-0321): "NetApp ONTAP Select is a software-defined storage (SDS) solution... deployed on your choice of commodity servers and hypervisors in your own data center." This source defines ONTAP Select as an on-premises SDS product, not a hybrid cloud consumption model.