Learn Polish: To rozwiązanie jest najbardziej popularne.

For longer adjectives and loanwords (words borrowed from other languages, like popularny), Polish often forms the superlative by putting najbardziej (the most) in front of the basic adjective. This is just like using "the most" in English! However, you might also hear the one-word version, najpopularniejsze, as both are correct.

Why do both To and popularne end in -e in this sentence?

They must match the gender of the noun they describe. Rozwiązanie is a neuter noun (it ends in -e). Therefore, we need the neuter word for "this" (to) and the neuter ending for the adjective (popularne instead of the masculine popularny).

The current lesson teaches us how to say "the best of...". How would I say "This solution is the most popular of all"?

You would add the phrase ze wszystkich at the end: To rozwiązanie jest najbardziej popularne ze wszystkich. We use the preposition ze instead of z because wszystkich (all) starts with a tough combination of consonants, making ze much easier to pronounce!

What if I specifically wanted to say "the most popular of the solutions"?