💡 Detailed Explanation
The correct choice is 'よう' (forming '病気のようです' - it seems he is sick). The grammar pattern '〜ようだ' (seems like / looks like) is used when the speaker makes a subjective conjecture based on reliable sensory evidence (in this case, '顔色が悪い' - looking pale). When connecting a noun to 'よう', 'の' is required: 'Noun + の + よう'. Sentence translation: 'Mr. Tanaka seems to be sick. Because he looks pale.' 'そう' (looks like) cannot connect directly with 'Noun + の' (it would be '病気そうだ' without 'の', and even then, '病気そうだ' is less common than '病気のようだ'). 'らしい' (hearsay/typicalness) connects directly as '病気らしい' (no 'の'). 'はず' means expectation/certainty ('病気のはずです' - he should be sick), but 'よう' represents the visual judgment much better here.