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