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The option that correctly tests for a specific platform build in Unity is denoted by the preprocessor directive `#if UNITY_ANDROID`. This directive checks whether the code is being compiled for the Android platform, which is managed by Unity's preprocessor directive system.

Using `#if UNITY_ANDROID`, Unity recognizes the specific platform during the compilation process, allowing developers to include or exclude blocks of code based on whether the build is targeting Android. This is useful for writing platform-specific code—essentially allowing different pieces of code to execute depending on the build target, which can be crucial for optimization and functionality on different platforms.

Other options presented do not match the valid predefined platform directives used by Unity. For instance, `#if PLATFORM_ANDROID`, `#if ANDROID_BUILD`, and `#if UNITY_BUILD_ANDROID` do not correspond to any recognized directives within Unity's scripting environment, and thus would not compile successfully when trying to conditionally include code based on the current platform. Therefore, the use of `#if UNITY_ANDROID` is the correct and standard way to check for Android builds in Unity projects.