Answer:
The speaker who does not contribute to the poem is:
D.the mother of the dead man
Step-by-step explanation:
Let's take a look at the poem:
Nobody heard him, the dead man,
But still he lay moaning:
I was much further out than you thought
And not waving but drowning.
Poor chap, he always loved larking
And now he’s dead
It must have been too cold for him his heart gave way,
They said.
Oh, no no no, it was too cold always
(Still the dead one lay moaning)
I was much too far out all my life
And not waving but drowning.
There are three speakers contributing to the poem: the narrator, the drowned man himself, and the friends of the dead (drowned) man. The narrator is the one letting us know that the dead man is laying, moaning:
Nobody heard him, the dead man,
But still he lay moaning
The drowned man is the one explaining that he was asking for help, not waving. His gesture was most likely misunderstood, which is why no one rescued him:
I was much further out than you thought
And not waving but drowning.
[...]
Oh, no no no, it was too cold always
[...]
I was much too far out all my life
And not waving but drowning.
His friends are only referred to as "they". As the man lies, dead, they talk about what a good person he was - which is a typical reaction of people in general when someone dies:
Poor chap, he always loved larking
And now he’s dead
It must have been too cold for him his heart gave way,
They said.
At no point the mother of the dead man is mentioned, nor does she contribute to the poem.