The word that can substitute the given group of words "One who offers one's services without being forced" is "Volunteer."

Explanation of the options:

☀ Skeptic: This word refers to a person who doubts or questions the truth or validity of something.

☀ Altruist: This word refers to a person who selflessly and unselfishly cares about the well-being and happiness of others.

☀ Volunteer: This word refers to a person who offers their services or time willingly and without being compelled or forced to do so.

☀ Scholar: This word refers to a person who is highly knowledgeable in a particular field of study or who pursues academic research and learning.