💡 Detailed Explanation
The Na-adjective '熱心' (nesshin) means enthusiastic, devoted, or passionate about a study, task, or interest. Option 1 is correct: 'Mr. Tanaka is very enthusiastic about studying Japanese.' Options 2 and 3 are incorrect because physical heat ('hot water' - お湯, or 'weather' - 天気) uses '熱い' (atsui) or '暑い' (atsui), not the metaphorical '熱心'. Option 4 is incorrect because catching a bad/severe cold uses 'ひどい風邪' (severe cold), not '熱心に'.