Golden Age of Fishes refers to the Devonian period, which lasted from approximately 419.2 to 358.9 million years ago. It was a period marked by the diversification and dominance of fish in the Earth's oceans and freshwater environments. Here is a detailed explanation of why the correct answer is option 'A':

1. The Devonian Period:
- The Devonian period is one of the geological periods in the Paleozoic Era, which spans from around 541 to 252 million years ago.
- It is often referred to as the Age of Fishes due to the significant evolutionary advancements and diversification of fish during this time.
- The Devonian period is divided into several stages: Early Devonian, Middle Devonian, and Late Devonian.

2. Fish Diversity and Dominance:
- During the Devonian period, fish evolved and diversified into various forms, occupying both marine and freshwater ecosystems.
- Jawed fish, known as gnathostomes, became more prevalent and diverse during this time.
- The Devonian saw the emergence of several major fish groups, including placoderms, acanthodians, and early sharks.
- Placoderms, armored fish with jaws, were particularly abundant and diverse during this period.

3. Evolutionary Advancements:
- The Devonian period witnessed significant evolutionary advancements in fish anatomy and physiology.
- Fish developed more efficient jaws, allowing them to feed on a wider range of prey.
- The development of paired fins provided better control and maneuverability in water.
- Some fish species developed lungs or lung-like structures, enabling them to breathe in oxygen-depleted waters or even venture onto land.

4. Ecosystem Impact:
- The diversification and dominance of fish during the Devonian period had a profound impact on ecosystems.
- Fish became top predators in many marine and freshwater environments, influencing the evolution and interactions of other organisms.
- The increased abundance of fish led to the evolution of more efficient predation strategies among other organisms.

In conclusion, the Golden Age of Fishes refers to the Devonian period, characterized by the diversification and dominance of fish. This period was marked by significant evolutionary advancements in fish anatomy and physiology, resulting in the emergence of various fish groups. The impact of fish on ecosystems during this time was substantial, shaping the evolution of other organisms.