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