Verse 29: Yuri Gagarin was the first person ________ into space.

Simplify relative clauses:

If the noun is preceded by the first, the second, the only….then we use to V (active); to be PP (passive) to reduce relative clauses

E.gl: He was the first person who came here.

=> He was the first person to come here. (active)

E.g2: I was the last person who was interviewed this morning.

=> I was the last person to be interviewed this morning. (passive)

Thus: Yuri Gagarin was the first person who traveled/ traveled into space. => Yuri Gagarin was the first person to travel into space.

=> Answer C (Yuri Gagarin was the first person to fly into space.)