The investigation described is a controlled experiment. The scientist is systematically manipulating an independent variable (the speed of the flowing water) to observe its effect on a dependent variable (the size of sediment particles carried away). This is done within a controlled environment (a trough) where other potential variables can be kept constant, allowing the scientist to establish a cause-and-effect relationship between water speed and sediment transport.

A. A field study would involve observing and collecting data in a natural, uncontrolled environment, such as a real river, not a laboratory trough.

C. A computational model would use computer simulations and mathematical equations to represent the process, rather than a physical setup with actual water and sediment.

D. An observational study involves measuring variables without manipulating them. Here, the scientist is actively changing the water speed, making it an experiment.

1. OpenStax, Biology 2e. (2018). Chapter 1: The Study of Life, Section 1.2: The Process of Science. Houston, TX: Rice University. In the "Testing Hypotheses" subsection, a controlled experiment is defined as a scientific test directly manipulated by a scientist to test a single variable (the independent variable) and observe its effect on another (the dependent variable). This aligns perfectly with the described scenario.

2. Grotzinger, J., & Jordan, T. (2014). Understanding Earth (7th ed.). W. H. Freeman. Chapter 6, "Sedimentation: Rocks Formed by Surface Processes," discusses how geologists use flumes (laboratory troughs) to conduct controlled experiments on sediment transport, varying factors like water velocity to understand deposition and erosion.

3. MIT OpenCourseWare. (2004). 12.110 Sedimentary Geology, Fall 2004. Lecture Notes 5: Fluid Flow and Sediment Transport. Massachusetts Institute of Technology. These notes describe the use of flume experiments to study how flow velocity (a controlled variable) influences the transport of sediment particles of different sizes, which is the exact methodology used in the question's scenario.