This puzzle is a mean-spirited parody of lazily-designed escape rooms!
PUZZLE 1: MORSE CODE
The Morse code decodes to "dotdashdotdotblankdashdashdashblank" etc. etc., which decodes to "longshortshortspaceshort" etc., which decodes to "dahditdah," so the answer is K.
PUZZLE 2: INVISIBLE INK
Any number of Photoshop-style manipulations will make the secret letter N in this image appear. Or you can turn your monitor at an extreme angle. Here's what it looks like if you mess with the exposure AND the brightness AND the contrast AND like four other things:
PUZZLE 3: FIND A THING
Scroll to the bottom of the page, past the link back to the puzzle list, to find the hidden letter O.
PUZZLE 4: A JIGSAW PUZZLE
Reassemble the pieces into the classic painting Landscape with the Fall of Icarus featuring a big letter B.
PUT IT ALL TOGETHER
The four sub-answers, K, N, O, and B, spell the answer to the puzzle, KNOB.