Question

which sentence breaks a grammar rule?
so many people would like to visit outer space that private companies are building rockets.
so often the price of a seat on the rockets helps pay for the development.
so long ago only highly trained astronauts were able to fly to space.
so that some average people will get to ride in those rockets.

Explanation:

The phrase "so that" is used to introduce a purpose clause, but it is incorrectly placed at the start of this sentence without a preceding main clause that establishes the action it is supporting. The other sentences correctly use "so + adjective/adverb + that" to show a cause-effect relationship, following standard English grammar rules.

Answer:

D. So that some average people will get to ride in those rockets.