Step 1: To strengthen a causal claim like "breakfast improves concentration", the best kind of evidence is a controlled comparison that isolates breakfast as the variable, since simple opinions or unrelated facts cannot establish cause and effect.
Step 2: Option A shows children merely enjoy breakfast, which is a preference, not a measure of concentration - it does not strengthen the claim.
Step 3: Option B shows breakfast is widely available, a fact about access with no connection to concentration outcomes - it does not strengthen the claim.
Step 4: Option C involves adults, not schoolchildren, and relies on a subjective "feeling" of alertness rather than a measured concentration outcome - it does not strengthen the specific claim about children.
Step 5: Option D provides a controlled trial with a comparison group and an objective concentration test, exactly isolating breakfast as the cause of the improved outcome - this is the strongest possible support.
Correct option: (4) In a controlled trial, children given breakfast scored higher on concentration tests than a similar group who skipped breakfast.