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