Answer :

it is least useful or effective in photosynthesis,,,
 because
the chlorophyll where light is trapped is itself green in colour...
so when sunlight falls on it ,, it traps all lights but not the green colour... and reflects green light so this green is not used or least used as it is reflected....
also plants appear green due to this reflection....

AL2006
Green is the least useful light for photosynthesis. We know that becsuse plants look green, so we know that the plant doesn't absorb green. It just discards it.