Google Cloud AI Platform offers tools and services for large-scale AI model development, training, and deployment. IBM Watson Developer Platform provides various AI and machine learning APIs and services for developers. Infosys Nia is an AI platform delivering cognitive capabilities and automation solutions for businesses. Alexa, Amazon's virtual assistant, utilizes AI and voice recognition for task execution. However, Alexa is not an AI development platform itself; it is an application powered by Amazon's AI technologies. Developers can create Alexa skills, but it is not a general AI development suite comparable to the other options. Therefore, the correct answer is option (D) Alexa AI.