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