Student or Learner
A dissertation or thesis is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings. In some countries/universities, the word "thesis" or a cognate is used as part of a bachelor's or master's course, while "dissertation" is normally applied to a doctorate, while in others, the reverse is true.
The term dissertation can at times be used to describe a treatise without relation to obtaining an academic degree. The term thesis is also used to refer to the central claim of an essay or similar work.
It will vary from speaker to speaker, and on the speed of the utterance and the formality of the conversation. It could be anywhere between clear /z/ and clear/s/. It's basically /z/, but if it's followed very closely by the voiceless /k/ of 'course', the /z/ may become devoiced, making it effectively the same sound as /s/.
In phonemic transcription, some people would transcribe this as /z/, considering the variation in this context to be merely an allophone of that phoneme. Others might use /s/, considering that a separate phoneme is produced here.