For the observation that establishes chlorophyll-a as the chief pigment in photosynthesis.

The question asks for the observation that establishes chlorophyll-a as the chief pigment in photosynthesis.

Let's evaluate each option:

• (A) Chlorophyll-a absorbs green light most efficiently, resulting in higher photosynthesis in green regions of the spectrum.

  • This statement is incorrect. Chlorophyll-a primarily absorbs blue and red light and reflects green light, which is why plants appear green. Photosynthesis is least efficient in the green region of the spectrum.

• (B) The rate of photosynthesis is highest at wavelengths where chlorophyll-a shows maximum absorption, particularly in the blue and red regions.

  • This statement is correct. The action spectrum of photosynthesis (the rate of photosynthesis at different wavelengths) closely matches the absorption spectrum of chlorophyll-a. Chlorophyll-a absorbs light most strongly in the blue-violet and red regions of the visible spectrum. The highest rates of photosynthesis are observed at these same wavelengths, indicating that chlorophyll-a is the primary pigment responsible for capturing light energy for photosynthesis.

• (C) Accessory pigments absorb light in the red and blue regions but do not transfer energy to chlorophyll-a.

  • This statement is incorrect. Accessory pigments (like chlorophyll-b and carotenoids) absorb light at different wavelengths and do transfer this energy to chlorophyll-a, broadening the range of light that can be used for photosynthesis.

• (D) Chlorophyll-a is the only pigment found in the chloroplasts of all plant cells, regardless of species or environment.

  • This statement is incorrect. While chlorophyll-a is essential and widely present, chloroplasts also contain other pigments, such as chlorophyll-b and various carotenoids, which are accessory pigments.

Therefore, the observation that establishes chlorophyll-a as the chief pigment is the strong correlation between its absorption spectrum and the overall rate of photosynthesis.

The final answer is $\boxed{\text{B}}$