**Explanation:**

In order to understand why a heat engine with 100% thermal efficiency violates the second law of thermodynamics, let's first define what thermal efficiency is and what the second law states.

**Thermal Efficiency:**
Thermal efficiency is a measure of how effectively a heat engine converts thermal energy into useful work. It is defined as the ratio of the useful work output to the heat input. Mathematically, it can be expressed as:

Thermal Efficiency = (Useful Work Output) / (Heat Input)

**Second Law of Thermodynamics:**
The second law of thermodynamics states that heat cannot spontaneously flow from a colder body to a hotter body. It also states that it is impossible to have a heat engine that operates in a cycle and converts all the heat absorbed from a hot reservoir into work without producing any waste heat.

**Explanation of the Answer:**
If a heat engine attains 100% thermal efficiency, it means that all the heat energy transferred to the engine is converted into useful work, and no waste heat is produced. However, according to the second law of thermodynamics, it is impossible to have a heat engine that operates in a cycle and converts all the heat absorbed from a hot reservoir into work without producing any waste heat.

**Implications:**
If a heat engine were to achieve 100% thermal efficiency, it would violate the second law of thermodynamics in the following ways:

1. **Perpetual Motion:** It would imply the creation of a perpetual motion machine of the second kind, which is not possible according to the second law.

2. **Violation of Kelvin-Planck Statement:** The Kelvin-Planck statement of the second law of thermodynamics states that it is impossible to construct a heat engine that operates in a cycle and produces no other effect except the extraction of heat from a single heat reservoir and the performance of an equivalent amount of work. A heat engine with 100% thermal efficiency would violate this statement by not producing any waste heat.

3. **Violation of Carnot Efficiency:** The Carnot efficiency is the maximum possible efficiency for a heat engine operating between two temperature reservoirs. It is given by the equation: Carnot Efficiency = 1 - (Tc/Th), where Tc is the temperature of the cold reservoir and Th is the temperature of the hot reservoir. If a heat engine had 100% thermal efficiency, it would imply that its efficiency exceeds the Carnot efficiency, which is not possible.

Therefore, a heat engine with 100% thermal efficiency violates the second law of thermodynamics.

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