Here is the full list of all 39 Return to Monkey Island achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Answered One Hundred Trivia Questions Correctly
Rescued Wally From Monkey Island
Found All The Copies of ‘At The End Of The Plank’
Found Cogg Island
Explored The Entire Ocean Floor
Got Hired With All Four Mop Heads
Answered Seventy-Five Trivia Questions Correctly
Told Everyone On Mêlée Island That You Are Looking For The Secret
Reached The End In 2 Hours Or Less
Returned To The World You Know
Started Part Five
Performed The Ole Switcheroo With The Replica Flag
Answered Fifty Trivia Questions Correctly
Became A Prize Chum
Let The Lumpsucker Escape
Gave Stan His Toothbrush
Did Gullet A Favor
Ahoy! More Than Twenty Times
Offered To Donate To The Earthquake Relief
Waited Patiently To See Carla
Decked Out The Sea Monkey II With Spooky Skulls
Started Part Four
Answered Twenty-Five Trivia Questions Correctly
Got To The Heart Of The Problem
Died For Real
Collected More Than Twenty Trivia Cards
Started Part Three
Braved The Scorched Alaska
Listed Every Mess On The First Swab’s Report
Answered Ten Trivia Questions Correctly
Swabbed The Hold Twenty Times
Started Part Two
Fulfilled Your Restroom Obligations
Attempted To Steal The Cook’s Mop
Thoroughly Examined All Of Wally’s Stock
Convinced Cobb To Tell You About LOOM
Freed Otis
Started Part One
Shared Your Luck