#### Answer

Please see the work below.

#### Work Step by Step

We know that:
1. According to Boyle's Law, volume is inversely proportional to pressure; hence, by tripling the pressure, volume decreases three times. Thus, figure (b) is the correct option.
2. As temperature doubles, the volume is also doubled according to Charles' Law. As the mass is constant and the volume is inversely proportional to density, the density is halved. The color of fluid shows density, so figure (a) is the correct option.
3. As more moles are added, the volume of the system will increase by the same factor as the mole. The mole ratio and volume ratio of gases are the same, and the density is the same as well, so figure (c) is the right option.
4. Temperature is halved, so volume is halved as well according to Charles' Law. According to Boyle's Law, when pressure is reduced by a quarter, volume increases four times. Thus, the volume is doubled, and figure (a) is the correct option.