Because the Dvorak keyboard puts vowels and other frequently used lett

The right answer is option C.

Errors are countable, therefore it is not appropriate to modify errors with the adjective less. Less is used to modify uncountable nouns. Fewer is the right adjective to modify a countable noun and only option C uses fewer instead of less. Options A, B, D, and E all use less, hence we can eliminate them based on this.

Option A: Because the Dvorak keyboard puts vowels and other frequently used letters right under the fingers on home row, where typists make 70% of their keystrokes, people can type 20% to 30% faster and make 50% less errors.

Option B: Because the Dvorak keyboard puts vowels and other frequently used letters right under the fingers on home row, where typists make 70% of their keystrokes, people can type making 50% less errors and 20% to 30% faster.

Option C: Because the Dvorak keyboard puts vowels and other frequently used letters right under the fingers on home row, where typists make 70% of their keystrokes, people can type 20% to 30% faster and make 50% fewer errors.

Option D: Because the Dvorak keyboard puts vowels and other frequently used letters right under the fingers on home row, where typists make 70% of their keystrokes, people can type faster (20% to30%), making errors less than 50% of the time.

Option E: Because the Dvorak keyboard puts vowels and other frequently used letters right under the fingers on home row, where typists make 70% of their keystrokes, people can type 20% to 30% faster while making 50% errors less. In additon to wrong choice of adjective for a countable noun, this sentence is awkward.