19Contemporary versions of transformational syntax require all NPs to have a case feature, even if they are not expressed inflectionally. This requirement is known as the .
(A) abstract case
(B) syntactic analysis
(C) case filter
(D) relational analysis

In this tutorial, we will discuss the Case Filter, which is the mechanism that regulates the distribution of noun phrases (NPs) within a phrase, primarily referring to Standard English.
 The Case Filter states that Every overt NP must be assigned case(Chomsky 1981).
More specifically, an NP cannot be licensed (may not appear) in a phrase unless it is marked with case.

Case is assigned to each NP in a phrase to show its function (labels an NP as the subject or object of a verb, object of a preposition, etc.) 
Case markers can be overt or covert and can be generally categorized as structural or non-structural.

http://www.linguisticsnetwork.com/syntax-the-case-filter/