Which of the following does not identify a unique date for which the celestial co-ordinates of the sun can be determined? vernal equinox full moon autumnal equinox summer solstice

Full Moon

Explanation:

Full Moon does not have a fix or defined date , like some other celestial bodies .

But the rest options , i.e. , Vernal equinox occurs at March 21 every year , similarly ,

Autumnal equinox - September 22  every year

and  

Summer solstice - June 21 every year ,

hence , the correct answer to statement given in the question is Full Moon .