I believe option C is wrong because it confuses the order of its clauses and it supposes that the likelihood will apply to the asteroid or comet at some point in the future.
For example, all of the following sentences sound correct to me:
Astronomers have developed a scale to rate how likely an outcome will be.
Astronomers have developed a scale to rate how likely to collide with Earth a particular asteroid or comet will be.
A particular asteroid or comet will be likely to collide with Earth.
However,
...
For example, all of the following sentences sound correct to me:
Astronomers have developed a scale to rate how likely an outcome will be.
Astronomers have developed a scale to rate how likely to collide with Earth a particular asteroid or comet will be.
A particular asteroid or comet will be likely to collide with Earth.
However,
...