“The invaluable nature of salvation” is the theme that this line from the passage hints at.
Salvation, or "being saved" means redemption from the power of sin. In practical terms, God's salvation is what we need to get to heaven or attain eternal life.
The correct answer between all the choices given is the third choice or letter C. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.