Hi Vaibhav,
"Progressively" means "in a gradual or step-by-step manner" and usually implies improvement or development happening continuously over time. While this word does fit the context of increasing, it doesn’t emphasize the regular passage of time as strongly as “year by year.”
For example, “progressively” could mean increasing monthly, weekly, or even randomly over time. It is less specific compared to "year by year," which clearly highlights annual progression

The other options don't fit as well because: "each year" and "annually" mean something happens yearly but lack the emphasis on continuous progression like "year by year." On the other hand, "progressively" suggests gradual improvement but doesn’t specifically indicate a yearly timeframe. "Year by year" is the best choice because it highlights a steady, consistent increase happening over each year while maintaining the natural flow of the sentence.

I hope this helps!