Transgenic animals are used to understand the contribution of genes in the development of diseases such as:

The correct option is Cancer and cystic fibrosis.

Step-by-Step Explanation:
1. Understanding Transgenic Animals: Transgenic animals are organisms whose DNA has been manipulated to possess and express an extra (foreign) gene. These animals are created using recombinant DNA technology to study gene regulation and their effects on the body's normal functions and development.
2. Role in Studying Diseases: Many transgenic animals are specifically designed to serve as models for human diseases. They are engineered to carry specific genes that make them susceptible to particular human illnesses, which helps scientists investigate how these genes contribute to the development of the disease.
3. Diseases Addressed: Today, transgenic models exist for many human diseases, which helps in analyzing the disease progression and testing new treatments. Typical examples of genetic and complex diseases studied using these models include:
• Cancer: Through oncogene insertion or tumor suppressor gene knockout.
• Cystic Fibrosis: By mimicking mutations in the CFTR gene.
• Alzheimer's Disease and Rheumatoid Arthritis.
4. Analyzing the Other Options:
• Cholera, typhoid, pneumonia, elephantiasis, ringworm, and kala-azar are infectious diseases caused by external pathogens (bacteria, fungi, worms, and protozoa) rather than diseases primarily investigated through human-gene targeting in transgenic animal models.