Why do brutus and cassius decide to continue their discussion in private?

a) they do not wish the soldiers to hear their discourse.

b) there is food and drink in the tent and they are both hungry.

c) they are afraid their plans may be overheard by spies.

d) all of the above.

The correct answer is A. they do not wish the soldiers to hear their discourse.
They are planning to kill Caesar, because they no longer think he is fit to rule the country. Thus, they move to a more secluded place to go over their plan, and they need to be alone so that the guards don't hear them and try to thwart their plan.