A receptor is a structure that detects stimuli. The stigma (eyespot) of Euglenoids detects , assisting with phototaxis. A. light intensity and direction B. vibrations C. chemicals D. sound frequency and direction E. temperature change

The correct answer is option A, that is, light intensity and direction.

Explanation:

A photoreceptive organelle witnessed in the motile or flagellated cells of unicellular photosynthetic species like euglenoids and green algae is known as the stigma or the eyespot apparatus. It permits the cells of the organism to detect the intensity and direction of the light and react towards it.  

This assists the organisms to either swim away from the light known as negative phototaxis, or to swim towards it known as positive phototaxis. The eyespots are the basic and the most typical eyes witnessed in nature. The eyespots comprise bright areas or orange and red granules of pigment and photoreceptors.  

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