Step 1: Understand the difference between 'advice' and its related forms.
Advice (with 'c') is a noun, meaning guidance or recommendations offered with regard to prudent future action.
We are looking for the verb form, which describes the action of giving advice.
Step 2: Evaluate each option.
(1) advise (with 's'): This is the verb form of 'advice', meaning to offer suggestions about the best course of action. For example, "I advise you to study hard."
(2) advisor: This is a noun, referring to a person who gives advice.
(3) advisable: This is an adjective, meaning to be recommended or sensible.
(4) advisory: This can be an adjective (e.g., an advisory committee) or a noun (e.g., a weather advisory), typically relating to giving advice or information. It is not the verb form of 'advice'.
Step 3: Conclude the correct verb form.
The correct verb form for the word 'advice' is 'advise'.
$$(1) advise$$