20分 高手請入F.3 physics (easy)

I think you have to understand first what "sensitivity" means.
Sensitivity, in a liquid-in-glass thermometer, is the change in liquid column for every degree of change in temperature. The unit used is commonly in "mm/C". Under such interpretation, the liquid column of a thermometer increases by, say 5 mm when the temperature rises by 1'C is more sensitive than another one with its liquid column increases only by 2 mm for the same one degree rise in temperature .

With this understanding in mind, it is clear that option A is entirely relevant to the sensitivity, as lengthening the capillary tube wouldn't help increasing the rise of the liquid column for a degree rise in temperature.
Option C, using a bulb of thicker wall would decrease the volume of the bulb cavity. Thus the amount (volume) of liquid that can be contained in the thermometer bulb is decreased. As expansion of a liquid, under a rise of temperature, depends on the amount of liquid stored in the bulb, the less liquid stored, the smaller will be the expansion. Therefore, a smaller expansion will lead to a lower rise of the liquid column (for the same degree rise in temperature), which implies that the sensitivity is decreased.