A smile the mother's face when the son walked onto the platform to receive the prize.
A. lit up
B. brought up
C. put up
D. took up
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(答案→)A. lit up
B. brought up
C. put up
D. took up
解析：A 考察动词短语辨析。句义：在儿子走上领奖台的是，母亲笑了一下，面露喜色。Light up脸上或者眼里流露出喜悦。Bring up养育，提出；put up建立，张贴；take up从事，占据。根据语境可知A正确。