The correct answer to the question is C. to discuss.

The sentence is asking for the expression or word that best completes the sentence. 

In this case, the sentence is talking about enabling delegates to do something with specialists in various fields.

The verb that fits best in this context is "to discuss. ".

Option A, "to discuss about," is incorrect because the correct phrase is simply "to discuss" without the word "about" following it.

Option B, "discussing," is incorrect because it is a gerund form of the verb and does not fit grammatically in the sentence structure.

Option D, "for discussing," is incorrect because it does not convey the intended meaning of the sentence.

Option E, "by discussing," is incorrect because it does not fit grammatically in the sentence structure and does not convey the intended meaning.

Therefore, the best option is C. to discuss.