The correct option is
A slithered, hissed, splashing
An onomatopoeia is 'the formation of a word from the sound associated with what it's named'.Option A: 'Slithered' imitates the sound of the action 'slither': to move smoothly over a surface with a twisting or oscillating motion "snake...slyly slithered past his left foot".
'Hissed' makes the sound of 'hissing': to make a noise like a long 's' sound "the snake hissed at the man".
'Splashed' makes the sound of the action 'to splash': strike or move around in a body of water "splashing into the water".
Hence, all these words are onomatopoeic and option A is correct.
Option B: 'Slyly' is an adverb that means 'in a cunning manner'. It does not imitate a sound.
Option C: 'Past' is also an adverb that means 'to move from one side to another'. It does not express sound.
Option D: 'Slyly' is an adverb and isn't onomatopoeic.
Hence, options B, C and D are wrong.