When practicing a speech for extemporaneous delivery, speakers should practice a. so they thoroughly memorize the entire speech. b. in stages. c. until they can give a perfect speech. d. after writing out the speech word for word

Option C  is the answer,until they they are able to give a perfect speech

Explanation:

Extemporaneous speech is a speech given with little or no preparation.

A speech that requires preparation but does not allow the speaker to  read from their write-up is also known as impromptu or extemporaneous speech.

From my explanation above, it is crystal clear that option A is out of context since at some time point extemporaneous is not prepared well ahead, the same applies to option D.

In order to be able to deliver anytime , the speaker should prepare for a wide range of topics such that they can handle most topic areas by preparing until they are able to give perfect speech