Answer
The fourth sulfur isotope is $^{32} S$.
Work Step by Step
Looking in the periodic table, we can see that the average weight for sulfur is equal to "32.06".
If the fourth isotope is $^{32} S$, the average weight for sulfur is between 32 and 36. If the fourth isotope is $^{35} S$, that will be between 33 and 36.
Since the periodic table weight is "32.06", the fourth isotope can't be $^{35} S$, because this number is not between 33 and 36. Therefore, the correct answer is $^{32} S$
