Answer:
A warrior frightens a gruesome monster away.
Explanation:
When you complete a sentence you have to make sure that the combination o verb and predicate are compatible, and that they express in the best possible way what you are trying to say. So let's analyze the answers:
1) frightens; away: this is grammatically correct, and both words combine well and add meaning to the message.
2) splits; in half: this is grammatically incorrect, it would have to be "into halves" or "in two"
3) kills; forever: kills already implies is forever, it is repetitive
4)beats; in pieces: beats doesn't imply that the monster was split into pieces, so these two words don't combine.