His first reaction was to widen his eyes in surprise when he saw the report card.

reaction 是名词，由动词 react（反应）加上名词后缀 -ion 构成。react 本身由前缀 re-（表示"再次"）和 act（行动）组成，字面意思是"对某事做出回应行为"。 reaction 在这里指"对刺激或情况的本能回应"，即"反应"。