This is a viral disease

The correct option is Measles.

To understand why this is the correct answer, let's analyze each of the options provided:
1. Rickets: This is a skeletal disorder caused by a deficiency of Vitamin D, calcium, or phosphate, not by a pathogen. Thus, it is a nutritional deficiency disease.
2. Measles: This is a highly contagious infectious disease caused by the measles virus (a member of the genus Morbillivirus in the family Paramyxoviridae). Since it is caused by a virus, it is classified as a viral disease.
3. Beri-beri: This is a nutritional disorder caused by a deficiency of thiamine (Vitamin B1), not an infectious agent.
4. Syphilis: This is a sexually transmitted infection caused by the bacterium Treponema pallidum, making it a bacterial disease.

Therefore, among the choices, Measles is the only viral disease.