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