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