Type conversion in Go involves converting variables from one type to another. When dealing with an interface{}, developers use type conversion to extract the underlying concrete type. This process is crucial for interacting with interfaces and accessing their specific functionalities. By employing type conversion, developers can ensure type safety and leverage the full capabilities of concrete types stored within interface{} variables, enhancing code clarity and maintainability.

The correct option is "To handle HTTP requests and map them to handlers". Routing in web development with Go involves directing incoming HTTP requests to the appropriate handlers or controllers based on the request URL.

Constants in Go are immutable, which means once their values are assigned, they cannot be altered or mutated throughout the program. This ensures predictability and reliability.

html/template is a built-in package in Go, specifically designed for generating HTML content. It provides strong security features like auto-escaping to prevent XSS attacks. Additionally, it offers better performance compared to text/template due to optimizations made for HTML. Ace and Mustache are also templating engines, but they lack the security features and performance optimizations provided by html/template.

The 'new' keyword in Go is used to allocate memory for a new variable. It initializes the variable with the zero value of its type and returns a pointer to the newly allocated memory. This is particularly useful when working with composite types like structs where the 'make' keyword isn't applicable.

The switch statement in Go is used to execute one of many blocks of code based on the value of an expression. It provides a clean and efficient way to handle multiple cases without excessive nesting.

Optimizing database indexes can significantly improve the performance of database operations, especially for frequently queried columns. Proper indexing reduces the time taken for data retrieval by enabling the database engine to locate records efficiently. It's an essential strategy for improving overall application performance.

In Go, mocking frameworks like "testify" or "gomock" provide utilities for creating mock objects. Mock objects are used to simulate the behavior of real objects in controlled ways during testing, allowing developers to isolate the code under test and verify its behavior.

The "testify" library in Go testing primarily serves the purpose of providing assertion functions. These assertion functions make it easier to write expressive and readable test cases by offering a variety of assertion methods for different types of assertions, such as equality, error checking, and more.

The advantage of using Gorilla Mux over the default HTTP router in Go is that it offers more features and flexibility. It allows defining complex route patterns with variables and constraints, making it suitable for large-scale applications.