In natural language acquisition, single words serve as complete sentences in the 1: holophrastic stage 2: silent period 3: telegraphic stage 4: overgeneralization

To answer this question, the only thing we must do is to analyze the very definition of the two early processes involved in natural language acquisition:

Holophrastic stage:

it is the child's language acquisition stage where the acquiring of syntax is mostly done by utterances of single words.

Telegraphic stage:

it is the child's language acquisition stage where the infant start uttering multiple words in an early stage.

so, to answer the question, in natural language acquisition, single words serve as complete sentences in the:

1: Holophrastic stage

answer: the first one

explanation:

explicitly is defined as “in a clear and detailed manner, leaving no room for confusion or doubt.” the rest are interpretive and not clearly proven leaving in fact possible “confusion or doubt” which explicitly, by definition cannot have.