Match List-I (Operons) with List-II (Number of structural genes) and select the correct answer:

List-I	List-II
a. Ara Operon of E. coli	1. 9 structural genes
b. His Operon of Salmonella typhimurium	2. 4 structural genes
c. Trp Operon of E. coli	3. 5 structural genes
d. Leu Operon	4. 3 structural genes

A. a-1, b-3, c-4, d-2

B. a-2, b-4, c-1, d-3

C. a-3, b-1, c-4, d-2

D. a-4, b-1, c-3, d-2

Answer: Option D