When a learner’s second language learning system seems to freeze or get stuck at some more or less deviant stage, that phenomenon is known as

The correct option is Fossilization.

In second language acquisition (SLA) theory, the term fossilization refers to the process in which incorrect or non-target language forms (deviant stages) become permanently fixed in a learner's interlanguage system. Despite further exposure, instruction, or practice, the learner seems to "freeze" or get stuck at this stage, and the errors become highly resistant to change.

This phenomenon was first introduced and defined by linguist Larry Selinker in 1972. It explains why many adult language learners retain a persistent accent, grammatically incorrect habits, or lexical errors even after years of living in and communicating in the target language environment.