Final answer:
The student using their understanding of the prefixes 'tele-' and 'graph-' to deduce the function of the telegraph was primarily demonstrating vocabulary skills.
Step-by-step explanation:
When the student saw the word telegraph and deduced that it is probably a machine that lets you write something to someone far away, they were using vocabulary skills. This is because the student has recognized the meaning of the word by breaking it down into its component parts (tele meaning distance and graph meaning write) and applying that knowledge to infer the function of the telegraph. This does not involve metacognition, which refers to thinking about one’s own thinking processes, nor does it necessarily involve critical thinking or inference in a broader sense.