What is inside a Hangeul block?

Correct answer: Letters are grouped into syllable blocks.

The examples combine consonant and vowel letters within a syllable. Since 가 has no final consonant, a final consonant is not compulsory.

Evidence: Hangeul letters represent sounds and are grouped into syllable blocks when written. The diagram and following paragraph distinguish letters from syllable blocks.

Correct answer: It fills the initial position without a consonant sound.

Here initial ㅇ fills a position without a consonant sound. That role cannot be generalised to final ㅇ.

Evidence: In 우유, the initial ㅇ in both 우 and 유 has no consonant sound. The second section explicitly limits the statement to the initial position.