Using HTTPS for transmitting API keys ensures that the keys are secured during transit. HTTPS (Hypertext Transfer Protocol Secure) encrypts the communication between the client and server, preventing unauthorized interception and ensuring the confidentiality of API keys.

Implementing Role-Based Access Control (RBAC) in APIs offers advantages such as fine-grained access control, which allows for precise permissions management. This enhances security by limiting access to sensitive data and functionality based on user roles. It does not necessarily simplify development but ensures secure access.

The primary purpose of HTTP (Hypertext Transfer Protocol) in Web APIs is to request and transmit data between clients and servers. It defines the methods for clients to request resources from servers and the format for the exchange of information. It plays a fundamental role in enabling communication and data transfer in web-based applications.

REST (Representational State Transfer) is an architectural style for designing networked applications. It emphasizes a stateless client-server interaction where resources are identified by URIs and can be manipulated using standard HTTP methods.

Implementing both real-time and historical monitoring for APIs is essential. Real-time monitoring helps in detecting issues as they occur, allowing for immediate responses. Historical monitoring, on the other hand, provides valuable data for analyzing long-term trends, identifying patterns, and improving overall API performance.

The purpose of API documentation is to guide developers on how to use the API effectively. It should include information on endpoints, parameters, request and response formats, and example use cases. Providing such detailed documentation is essential for developers to understand and integrate with the API.

HTTP status codes in the range "200-299" indicate successful API requests. These codes signify that the request was received, understood, and successfully processed.

gRPC is preferred over traditional REST APIs when high performance and low latency are essential. gRPC uses HTTP/2, which supports multiplexing and binary protocols, making it more efficient in scenarios that require fast and responsive communication. It's particularly well-suited for microservices and real-time applications.

The SOAP architectural style for APIs uses service interfaces defined in a machine-processable format like WSDL (Web Services Description Language). SOAP is known for its strict standards and protocols for communication, making it suitable for enterprise-level applications.

GraphQL optimizes performance by allowing clients to specify custom queries, which include only the data they need. This reduces over-fetching of data that is common in traditional REST APIs. By efficiently selecting data, GraphQL can minimize the amount of unnecessary data transferred over the network, resulting in faster response times and reduced server load. Caching and request batching can further enhance performance.