I wonder if, when taking the official test, which syntax should we use Mongo 3.6, Mongo 4.0, Mongo 4.2 ?
Could you talk a little more about what you mean by
As per my knowledge, the syntaxes are same for all the version.
MongoDB certification exam is version specific so whenever you writing the exam - answer the questions based on the MongoDB version of the test.
Some examples with their respective links
You can read clearly : “new syntax” .
Link : https://docs.mongodb.com/manual/release-notes/4.2/
In some versions the option is available and in others not
Depending on the version, the behavior is different.