3)  Which of the following quantities must be measured in order to
    determine the mass of Mars?
    (a) The radius of Phobos' orbit around Mars
    (b) The radius of Mars' orbit around the Sun
    (c) The period of Phobos' orbit around Mars
    (d) The period of Mars' orbit around the Sun
    (e) The period of Mars' rotation about its axis

(a) and (c) are correct. To determine the mass of an astronomical object, you need to measure the period and semimajor axis of the orbit of something which orbits that object. Phobos orbits Mars (and its orbit is quite circular, so the semimajor axis of the orbit is the same as its radius). (b) and (d) are the things you might measure in order to determine the mass of the Sun, since that is what Mars orbits. (e) is irrelevant, since the length of the Martian day has nothing to do with the mass of Mars.