Under certain conditions, enthalpy is the energy released or absorbed by the reaction, so assuming we are under these conditions, we can complete the first blank with "energy", since we are taking about enthalpy in a reaction.
This energy is normally in the form of heat, so we can think of this energy as heat going in or out of the system of reaction. The only option that fits the second blank is "passes", so the heat that is passing into or out of the "system", that is the third blank;
When the enthalpy of the system increases, it means that the reaction neede more energy, it absorbed energy, so energy is added to the reaction, thus, the fourth blank is "increases".
Similarly, the energy going out of the reaction make the system lose enthalpy, so the fifth blank is "decreases".
So, the blanks, in order, are:
- energy
- passes
- system
- increases
- decreases