Question

Indeed, I ___I made more progress in German than in any of my other studiesa) think b) thinking c) to thickd) decide​

Answer 1

B) think.
Indeed. I think I made more progress in German than in any of my other studies

Answer 2

The correct answer is "a) think."

So the complete sentence would be: "Indeed, I think I made more progress in German than in any of my other studies."