日本語は勉強___ほど面白くなります。
Q. Which form completes the shortened 'the more you study...'?
Translation
The more you study Japanese, the more interesting it gets.
Explanation
Besides すればするほど, the dictionary form + ほど alone also works: 勉強するほど面白くなる. Directly before ほど the plain form is required — すれば cannot attach there.