This is “Canalave City” from the RPG “Pokémon Diamond/Pearl/Platinum” released by The Pokémon Company.
The music is composed by Go Ichinose.
This music is played in a harbor town. The arpeggio at the beginning reminds me of the waves that come and go.
In the Platinum version, it is famous as the place where you can get the Pokémon called Darkrai.
In parts A and A’, the bass goes down a semitone at a time in a smooth flow while using the inverted form.
As for the arrangement, in part A, I used only the melody and arpeggio as the beginning of the song to create a quiet feeling.
And in part A’, I added the harmonies that I heard from the original song to create an uplifting feeling.
Part B modulates from the original key of A major to C major, which is minor 3rd higher.
This is relatively natural modulation since it is “relative key of parallel key” from the point of view of the original key.
The chord progression used for the modulation is II-V-bVII (=V in C major).
I happened to find on a page explaining the modulation I refer to, this chord progression is same as the one from Beatles’ “Here, There And Everywhere”, when it enters the chorus.
I’ve picked up only the most prominent phrases for me, although there are actually some arpeggios in this part.