Final answer:
Carmelo Casas is the architect who left the firm after 20 years.
Step-by-step explanation:
The architect who left the firm after 20 years was d. Carmelo Casas. This firm is well-known for its various architectural projects throughout the Philippines, such as the Philippine Plaza, ABS-CBN Complex, and the Pacific Plaza Towers. It is also recognized for being awarded the firm of the year by the Philippine Institute of Architects in 1997. Carmelo Casas' departure marked a significant change in the firm's dynamic.