a. their pigments

b. excretion of coloured substances

c. formation of coloured chemicals in water facilitated by physiological degradation of algae.

d. absorption of light by an algal cell wall.

Answer: a. their pigments

Explanation: The algal blooms occur when natural conditions allow the explosive growth of phytoplankton which creates very dense clouds of these organisms that can change the color of seawater. All photosynthetic phytoplankton has chlorophyll as its primary pigment, which explains why researchers measure chlorophyll concentrations to determine how much phytoplankton is in a particular region. Phytoplankton also has access colors that allow them to use different lengths of sunlight in cellular growth and diet.