Question

the mean length of utterance (mlu):

is a good predictor of childrens grammatical abilities.
directly correlates with age: all typically developing children are at the same mlu at the same age.
is a simple way to describe childrens grammars.
is not a very good predictor of childrens grammatical abilities.

Explanation:

The Mean Length of Utterance (MLU) is a well - established measure in language development research and is considered a good predictor of children's grammatical abilities as it reflects the complexity of their language use. It doesn't mean all children of the same age have the same MLU as there is individual variation, and while it gives an indication of grammar, it's not a simple description of children's grammars.

Answer:

The Mean Length of Utterance (MLU) is a good predictor of children's grammatical abilities.