One full day is 24 hours.


24 hours is  how many half-lives ? - It is   = 2.82 half-lives, i.e. close to 3 half-lives.


Hence, after 24 hours, the remaining amount will be  about   =   of the initial amount, which is 20 ounces.


So, we can expect  something about  = 2.5 ounces for remaining mass.


The closest option is D).    ANSWER


All other optional masses are much lesser and much further in time than 3 half-lives; 
so, this MENTAL estimation is just enough to make a final conclusion.

If -life consists "a" time periods, the DECAY CONSTANT, or 

                                                So, we have:                                          
                                                             

                            OONTINUOUS GROWTH/DECAY formula: 
                                                 So, we get:  --- Substituting  

Amount remaining after 1 full day (24 hours): A = 2.82168 = 2.82 (ROUNDED to 2 decimal places) ounces (CHOICE D).