Instana's Synthetic monitoring for API tests utilizes a Node.js runtime environment. Consequently, the scripts for these tests are written primarily in JavaScript. This allows users to create complex, multi-step API test scenarios, validate responses, and handle authentication using a widely adopted and versatile scripting language. The environment provides various pre-packaged Node.js modules to facilitate the creation of these test scripts, such as modules for making HTTP requests and assertions.

A. Java: While the Instana backend and agent have components written in Java, it is not the language used for writing Synthetic API monitoring scripts.

C. Python: Python is a common language for scripting and testing but is not the language used by the Instana Synthetic API test runtime environment.

D. Go: The Instana agent is written in Go, but this language is not used for user-defined Synthetic API scripts within the platform.

1. IBM Instana Documentation - API script tests: "API script tests are written in JavaScript and run in a Node.js environment. You can use API script tests to make a sequence of API requests." This section explicitly states JavaScript as the language. (Source: IBM Documentation, Instana, "Synthetic monitoring > API script tests").

2. IBM Instana Documentation - Scripting API tests: This page details the JavaScript-based methods and modules available for scripting, such as $synthetic.execute() and the use of the got HTTP request library, which are all JavaScript constructs. (Source: IBM Documentation, Instana, "Synthetic monitoring > Creating an API script test > Scripting API tests").

3. IBM Instana Documentation - API script reference: The reference guide for scripting lists available modules and their APIs, such as assert, btoa, crypto, and got. These are all standard or specific Node.js/JavaScript libraries, confirming the underlying language. (Source: IBM Documentation, Instana, "Synthetic monitoring > API script reference").