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