Which hypothesis states that grammatical structures are acquired in a predictable order?

The Natural Order Hypothesis explains that the acquisition of grammatical structures occurs in a predictable sequence, which is not influenced by explicit teaching but rather follows a natural progression as learners acquire a second language. This hypothesis was introduced by Stephen Krashen as part of his larger theory on second language acquisition.

It emphasizes that regardless of the learner's first language or the instruction they receive, certain grammar structures tend to be learned before others. For example, simpler structures usually precede more complex ones, suggesting that there is a kind of internal processing order that all learners share as they become proficient in a new language.

In contrast, the other hypotheses mentioned focus on different aspects of language acquisition. The Acquisition Theory pertains to the overall understanding of how languages are learned, the Input Hypothesis addresses the role of comprehensible input in language learning, and the Affective Filter Hypothesis talks about emotional factors that can either enhance or hinder language acquisition. The Natural Order Hypothesis specifically targets the sequence of grammatical structure acquisition, making it the correct answer.