Identify the onomatopoeic words in the following sentence:

 The snake hissed at the man and slyly slithered past his left foot before splashing into the water. 

Onomatopoeia is defined as "the creation of a word from the sound connected with what it is termed."

Option A: 'Slithered' mimics the action 'slither': to travel smoothly along a surface with a twisting or oscillating motion "snake...slyly slithered past his left foot."

'Hissed' sounds like 'hissing': to create a noise that sounds like a long sound "the snake hissed at the guy."

The word 'splashed' sounds like the verb 'to splash': to strike or move around in a body of water "splashing into the water."

As a result, all of these words are onomatopoeic, and choice A is correct.

Option B: 'Slyly' is an adverb that indicates 'in a devious way.' It does not mimic any sound.

Option C: The adverb 'past' also indicates 'to shift from one side to another.' It does not convey sound.

Option D: 'Slyly' is an adverb, not an onomatopoeic word.

As a result, alternatives B, C, and D are incorrect.