**Core Concept**
The use of combination anti-tubercular drug therapy is a cornerstone in the treatment of tuberculosis (TB). This approach is based on the principle of exploiting the differences in the growth rates and metabolic states of various bacteria, making it more difficult for resistant strains to emerge.

**Why the Correct Answer is Right**
The primary purpose of combination therapy is to prevent the emergence of drug-resistant TB strains. When multiple drugs with different mechanisms of action are used together, it becomes challenging for bacteria to develop resistance to all of them simultaneously. For example, isoniazid and rifampicin, two commonly used first-line antitubercular drugs, target different enzymes involved in DNA synthesis and transcription, respectively. By using these drugs in combination, the likelihood of resistant strains emerging is significantly reduced.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because monotherapy, or using a single drug, is more likely to lead to the development of drug-resistant TB strains.
* **Option B:** This option is incorrect because although reducing the treatment duration may be beneficial, it is not the primary purpose of combination therapy.
* **Option C:** This option is incorrect because while combination therapy may have some effect on reducing the risk of relapse, it is not the primary purpose of this approach.

**Clinical Pearl / High-Yield Fact**
The combination of isoniazid and rifampicin is often referred to as the "first-line" treatment for TB due to its effectiveness in preventing the emergence of resistant strains.

**Correct Answer: D. Reducing the emergence of drug-resistant TB strains.**