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