The correct option is Option D, which states that Fasciola hepatica (liver fluke) infects its intermediate host and primary host at the larval stages of miracidium and metacercaria respectively. Here's a brief overview of the life cycle of Fasciola hepatica to explain why:

1. Miracidium: The life cycle begins when eggs released into the environment from an infected primary host (typically a sheep or a cow) hatch into miracidia upon contact with water. The miracidium is equipped to infect the intermediate host, which is a specific type of freshwater snail.

2. Redia & Cercaria: Inside the snail, the miracidium undergoes several developmental stages, initially transforming into sporocysts, then into rediae, and finally into cercariae. The redia stage is particularly noteworthy because within the snail, it will produce several generations, which increase the number of cercariae produced. The cercariae are then released from the snail into the water.

3. Metacercaria: Once free-swimming, the cercariae encyst as metacercariae on aquatic vegetation or other surfaces. This encysted stage is infectious to the primary host. When the primary host ingests these metacercariae-laden plants or water, the life cycle progresses.

4. Adult: Inside the primary host's intestines, the metacercariae excyst and migrate to the liver, where they mature into adult flukes, completing the cycle.

Therefore, the miracidium phase targets the intermediate snail host, initiating internal development, while the metacercaria phase represents the stage at which Fasciola hepatica is capable of infecting the primary host, marking the transition from external to internal parasitism in the latter.