Explanation:

- Option 1: "is relax"

- The word "relax" is in its base form and does not fit grammatically after "is." It should be in the form of a gerund or infinitive with "to."

- Option 2: "is relaxed"

- "Relaxed" is an adjective and does not correctly describe an action or idea. It would describe a state of being which doesn't align with the action of being on holiday.

- Option 3: "is to relaxing"

- This is grammatically incorrect. It mixes the infinitive "to" with the gerund "relaxing," creating an incorrect structure.

- Option 4: "is relaxing"

- This option is correct. "Relaxing" serves as a gerund, which can act as a noun related to the act of relaxing, making the sentence grammatically correct.