Only D doesn't Weaken the Argument

C
more by process of elimination.
Not D because, if a celebrity advertises for another rival as well, how can a particular ad carry celebrity sale power.
All other options are weak as well.

I personalized the argument--judge as if you are a consumer or the company.

Going for C....
whats the OA
_________________

Regards,
Invincible...
"The way to succeed is to double your error rate."
"Most people who succeed in the face of seemingly impossible conditions are people who simply don't know how to quit."

Well...Not sure of...whether I can convice u...lets see...

A) A survey shows that 90% of members of the public have never heard of Joan Fern.
Incorrect: This will weaken the argument as If people never heard of Joan Fern, No one will be enticed by this feature.

B) One of the celebrities listed admitted in a newspaper article that he/she had never heard of InterNex.
Incorrect: If the Internex Claims that a Particular celeb is associated with them, and the argument is refuted by the same celeb, then this may have a neg impact over programme success.

C) The people at whom this advert is targeted would probably never use the phrase "Back of the Net".
Correct: Note that the option says "Probably never use the phrase..." well.....if the audience like the programme, but dont want to use the phrase..its immaterial....Right !! We have caught hold of customer....isnt it. Now even if they dont use.....no harm done.....

D) Craig Boone provides services publicly for a rival Internet service.
Incorrect: This can be a contender, but assuming this to be true, this will only lessen one of the attractive features of adv. Option C has less harm as compared to this.

E)The people at whom the advert is targeted are not generally interested in football or debating ethics.
Incorrect: Here, two areas are eliminated..

We have to find the option Least Weakening....C < D < E....

Hope this solves the problem...
_________________

Regards,
Invincible...
"The way to succeed is to double your error rate."
"Most people who succeed in the face of seemingly impossible conditions are people who simply don't know how to quit."