In the given sentence we would be understanding the meaning of the word "momentous" and then pick out the word which is nearest to the meaning.
Momentous means of great importance or significance, especially in having a bearing on future events. This means the correct option is "important"
If we look into the other options,
transient means lasting only for a short time; impermanent.
Wrong means not correct or true; incorrect
Hopeless means feeling or causing despair
Option B is correct