The sentence tells us that the people 'should' start work at 8:15 but they do not. Thus, the answer must mean 'should'.
Option A is incorrect. Meant here means 'destined'. Hence, it does not fit.
Option B is correct. Supposed is closest in meaning to should. So, it is the best fit for the blank.
Option C is incorrect. Scheduled means 'planned'. They are not planning to work but, they should be working. Thus, C does not fit.
Option D is incorrect. Made in this context can mean 'forced'. Since they are not being forced to work, D is not suitable.