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