What is the foreign plural form of synthesis?

There isn't a single, universally accepted foreign plural form of "synthesis". However, depending on the specific language and context, some possible options include:

Latin:

* syntheses (most common and widely accepted)

* synthesin (archaic)

Greek:

* syntheseis (less common, but used in some scientific contexts)

Ultimately, the best plural form to use would depend on the specific language and context. If you're unsure, it's always best to consult a dictionary or grammar guide for the appropriate plural form.