Final answer:
The statement of opinion among the provided options is a: 'All students should keep informal writing out of their schoolwork,' as it expresses a judgment rather than a verifiable fact.
Step-by-step explanation:
In determining which of the given options is a statement of opinion, we should look for a sentence that conveys a personal belief, view, or judgment that is not based on direct evidence or knowledge. Option a, 'All students should keep informal writing out of their schoolwork,' is a statement of opinion because it contains the phrase 'should keep,' which indicates a recommendation or judgment rather than a factual claim.
In contrast, options b, c, and d provide specific statistical data or information that can be verified or falsified, and therefore, they are statements of fact. For example, 'Half of the teens surveyed failed to use proper punctuation in schoolwork' (option b) and 'Twenty-five percent of students have used emoticons in their formal writing for school' (option d) present clear, measurable figures.