From left to right, the three regions of a phase diagram represent the of a substance.
a gas phase, liquid phase, and solid phase
b melting point, triple point, and boiling point
c melting point, boiling point, and sublimation point
d solid phase, liquid phase, and gas phase

D solid phase, liquid phase, and gas phase

Explanation: The phase diagram is  a graphical representation of the physical states of a substance under different conditions of temperature and pressure. The temperature is usually plotted on X axis and pressure is plotted on Y axis.

Thus as we increase the temperature and move from left to right the phase will change from solid and liquid as the kinetic energy of the molecules will increase and on further increasing the temperature, the liquid phase will change to gaseous phase as molecules will gain more kinetic energy on further increasing the temperature.

catya your not at all which answer i've seen you do this on another question!

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.