Final answer:
Kant's synthetic a priori refers to necessary truths based on empirical experience. Option c is not an example of Kant's synthetic a priori as it pertains to the unknowable nature of objects.
Step-by-step explanation:
Kant's synthetic a priori refers to propositions that are both synthetic (based on empirical experience) and a priori (known independently of experience). They are necessary truths that go beyond what can be observed directly. Examples of Kant's synthetic a priori include mathematical propositions like '2+2=4' and 'all triangles have three sides'.
So the answer to the question is option c. 'Object as they are in themselves is unknowable to us' is NOT an example of Kant's synthetic a priori because it pertains to the unknowable nature of objects, which does not directly involve empirical experience.