Question

name one scientific tool kellen needed to use to complete his observations, and explain how he used this tool to complete his work.
what unit of measurement did kellen use?

Explanation:

Since the question is about a scientific tool for observations and its usage, common scientific tools like a microscope (used to observe tiny objects), a telescope (for celestial observations), a thermometer (to measure temperature during observations), etc. could be considered. Without more context about Kellen's work, we can't be more specific. But we can give a general - type answer. For example, if Kellen was observing plant cells, he might use a microscope. He would place a prepared slide of plant cells under the microscope, adjust the focus using the focus knobs, and then view the cells to make his observations.

Answer:

One possible answer could be a microscope. He placed a sample (such as a plant or animal tissue sample on a slide) under the microscope, adjusted the objective lenses and focus knobs to get a clear view, and then made his observations of the microscopic structures.