**Explanation:**

The correct answer is option **c) Indulge**.

**Gratify** means to give pleasure, satisfaction, or contentment to someone. It is often associated with fulfilling desires or wishes. Let's understand why option **c) Indulge** is the correct answer and why the other options are incorrect:

- a) **Appreciate**: While this word can sometimes be used synonymously with gratify, it does not capture the sense of indulgence or fulfilling desires. Appreciating something means recognizing its value or quality, but it does not necessarily involve satisfying someone's desires.

- b) **Frank**: This word means to be open, honest, or straightforward in speech or behavior. It does not have the same meaning as gratify and is not a suitable synonym.

- c) **Indulge**: This word perfectly captures the meaning of gratify. To indulge someone means to allow them to have or do something they want, especially if it is considered indulgent or excessive. When you gratify someone, you are indulging their desires or wishes, bringing them pleasure or satisfaction.

- d) **Pacify**: While this word can sometimes be related to gratify, it does not fully capture the meaning. Pacifying someone means to calm or soothe them, often by resolving conflicts or reducing their anger or anxiety. It does not necessarily involve satisfying desires or bringing pleasure.

In conclusion, option **c) Indulge** is the correct synonym for **gratify** because it accurately captures the meaning of fulfilling desires or wishes, bringing pleasure or satisfaction to someone.