Which of the following is a messaging system used in financial trading?

The correct choice is the messaging system specifically designed for financial trading, which is Thomson Reuters. This platform is widely recognized in the financial industry for providing real-time financial data, news, and trading capabilities. Thomson Reuters is integral to many trading operations, offering specialized messaging services that facilitate secure and efficient communication among financial professionals.

Other options, while they are messaging systems or platforms, do not have the same level of industry focus or tailored features for the financial trading environment. E-mail is a general communication tool widely used across all sectors but lacks the specialized functions needed for trading. Slack and Google Chat are popular messaging platforms that serve various industries but are not specifically designed for financial trading communication. They do not provide the depth of financial data integration, security, and functionality that Thomson Reuters does, making it the most relevant choice for the context of this question.