The word "diversity" refers to the presence of a variety of different elements or qualities. To find the most appropriate antonym for "diversity," we should look for a word that signifies a lack of variety or the state of being uniform or consistent.

Option 1: "Melancholy" - This word refers to a feeling of deep sadness or sorrow and is unrelated to the meaning of "diversity."

Option 2: "Equilibrium" - This word means a state of balance or stability and doesn't provide an antonym for "diversity."

Option 3: "Affinity" - This word means a natural liking or attraction to something or someone and is also unrelated to the meaning of "diversity."

Option 4: "Uniformity" - This word is the most appropriate antonym for "diversity." "Uniformity" means the state or quality of being uniform, consistent, or lacking in variety. It directly contrasts with the concept of diversity, as it suggests a lack of diversity or variety.

Therefore, the most appropriate antonym for "diversity" among the options provided is "Uniformity" (Option 4).