A subject is the word, usually in a form of nouns, that is being talked about in the sentence. A subject can either be simple or compound. In the given sentence above, the subject is the word "we". And "we" is the direct doer of the action word "invite".
The answer to this question would be b
You did not include options in your question, but based on the meaning of the word cryptic, your answer will be 'A HIDDEN MEANING'.
When you say that something is cryptic, it means that, that thing has a meaning that is mysterious or obscure.
From the way the word was used in the given sentence, it means that the message the teacher was passing across about the test is hidden, it was not understood by any of the students.