Final answer:
Evolutionary trend generally involves an increase in complexity (option C), as exemplified by the transition from green algae to more complex plants.
Step-by-step explanation:
The evolutionary trend for increasing organismal complexity can be best described as an ongoing process that generally involves an increase in complexity, option C. When examining the history of life on Earth, we can see that there is a pattern of evolution where simple organisms gradually diversify and develop into more complex forms. For instance, the transition from single-celled green algae to complex multicellular plants illustrates this increase in complexity. Green algae, while simple, posses the ability to perform photosynthesis and can be seen as a foundational organism that through evolutionary processes, such as natural selection and mutation, gave rise to more complex plant structures and forms, exemplifying the evolutionary trend toward increased complexity.