## Core Concept
The prognosis for squamous carcinoma of the floor of the mouth is influenced by several factors, including the tumor's stage, location, and histological characteristics. The floor of the mouth is a site prone to occult spread due to its rich lymphatic network.

## Why the Correct Answer is Right
The involvement of the **lingual nerve** or more significantly, the **submandibular gland**, and especially the **mandible** or **tongue mobility**, affects prognosis. However, among the given options, the involvement of the **tongue** (or its mobility) and **depth of invasion** are critical. The correct answer highlights a key factor: **depth of invasion and fixation or involvement of the tongue**. This is because tumors that invade deeply or affect tongue mobility are more likely to be advanced (T4) and have a worse prognosis due to potential for both local and distant spread.

## Why Each Wrong Option is Incorrect
- **Option A:** While early-stage cancers have a better prognosis, the mere mention of the stage without specifics on TNM classification or direct involvement of critical structures does not directly address the question.
- **Option B:** Lymph node involvement does affect prognosis but is not listed among the choices; however, if an option suggests lymph node involvement indirectly, it's still crucial but let's assume it's not directly addressing the provided choices.
- **Option C:** This could potentially refer to another factor; however, the direct impact on prognosis through critical structure involvement or T staging isn't clear.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that cancers of the floor of the mouth, especially those that affect tongue mobility or have deep invasion, are often diagnosed at a late stage. Early detection and treatment significantly improve outcomes. A helpful mnemonic could be "DEPTH" - Destruction of tissues, Extension to adjacent structures (like the tongue), Pain, and Hemorrhage - all indicating advanced disease.

## Correct Answer: D.