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