Why is this the answer?

The answer is syllable because this tricky riddle is actually asking you to count the number of sound-parts in each word it uses! A syllable is like a little beat in a word. If you clap while you say a word, each clap is a syllable. Let's try it with the words in the riddle: t- The word 'He' has one beat: He (1 syllable). t- The word 'person' has two beats: per-son (2 syllables). t- The word 'citizen' has three beats: cit-i-zen (3 syllables). t- The phrase 'human being' has four beats: hu-man be-ing (4 syllables). t- The word 'personality' has five beats: per-son-al-i-ty (5 syllables). t- And the phrase 'inhabitant of earth' has six beats: in-hab-i-tant of earth (6 syllables). See how the number of syllables in each word or phrase matches the numbers in the riddle? That makes "syllable" the perfect answer. This is a very clever word puzzle that makes you think about how words are put together.