**Core Concept**
The question is testing the understanding of the causes of green discoloration on the surface of teeth, which is a common dental complaint. The underlying principle involves the interaction between dental enamel and external substances.

**Why the Correct Answer is Right**
The correct answer is due to the presence of **fluorosis**. Fluorosis occurs when there is an excessive intake of fluoride during tooth development, leading to the deposition of fluorapatite in the enamel. This can cause a range of dental abnormalities, including white or brown spots, and in severe cases, a greenish discoloration on the surface of teeth. The mechanism involves the substitution of hydroxide ions in hydroxyapatite with fluoride ions, resulting in the formation of fluorapatite.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because **tetracycline staining** typically causes a brown or yellow discoloration on the surface of teeth, rather than a greenish hue.
**Option B:** This option is incorrect because **smoking** can cause a range of oral health problems, including bad breath and gum disease, but is not directly associated with green discoloration on the surface of teeth.
**Option C:** This option is incorrect because **poor oral hygiene** can lead to a range of dental problems, including tooth decay and gum disease, but is not a direct cause of green discoloration on the surface of teeth.

**Clinical Pearl / High-Yield Fact**
It's essential to note that excessive fluoride intake during tooth development can lead to fluorosis, which can cause a range of dental abnormalities, including green discoloration on the surface of teeth. This highlights the importance of monitoring fluoride intake, especially in children.

**Correct Answer: D. Fluorosis**