Which word describes an individual who is firm and steadfast?

The term "staunch" is used to describe someone who is firm, steadfast, and loyal in their beliefs or opinions. This word conveys a sense of unwavering support and reliability, implying that the individual does not easily waver in their convictions or loyalty, regardless of circumstances. This steadfastness can be applied in various contexts, such as personal beliefs, political views, or support in relationships.

In contrast, the other options do not capture this quality of firmness or steadfastness. "Sodden" denotes being soaked or heavy, often lacking energy or vitality. "Serene" refers to a state of calmness and tranquility but does not imply a sense of strength or determination. "Sluggard" describes a person who is habitually lazy or idle, which is antithetical to the concept of being firm or steadfast. Therefore, "staunch" is the appropriate choice to describe someone who embodies those qualities.