Which light range is least effective in photosynthesis

Green light (≈500–570 nm) is generally least effective for photosynthesis in typical green plants because chlorophylls reflect/transmit green wavelengths and absorb blue (430–470 nm) and red (640–680 nm) more strongly. 

Hence, action spectra peak in blue and red, and leaves appear green. Add a nuance: while least effective per photon at the leaf surface, some studies note green light penetrates deeper into leaf canopies and can contribute to photosynthesis in lower layers—useful context if asked about controlledenvironment lighting. 

For school/board exams, stating “Green is least effective; Blue/Red most effective” is sufficient.