CrossTalk uses the latest released version of .NET at the time of release. .NET however is backwards compatible and can load any .NET library of the current version, or any older version.
For example, if the CrossTalk release you are using links to .NET 4.6.2, you can use any .NET library for any .NET version from 1.1 to 4.6.2. In some cases for very old .NET versions, some settings may be required in a .config file to accompany your application.