Final answer:
The correct answer is 'categories:' as it properly introduces a list of items in the given context, following the conventions of Standard English.
Step-by-step explanation:
The choice that completes the text so that it conforms to the conventions of Standard English is 'categories:' (Option A). This punctuation is appropriate because it introduces a list of two items that are explained further in the text: pictographs and petroglyphs. The colon serves as a marker that the reader can anticipate a clarification or enumeration to follow.