Answer: d. "until my body leaned, gleamed, hardened / into a core of speed"
Explanation: This detail conveys a sense of accomplishment and success. The use of words like "leaned," "gleamed," and "hardened" suggests physical and personal growth. It indicates that the speaker's body has transformed into a powerful and strong force, symbolizing their achievement in swimming. The phrase "into a core of speed" further emphasizes the speaker's triumphant development and mastery of swimming skills.