Some of the sentence are grammatically correct and some are incorrect. Find which part of the sentence has an error and mark that part. If there is no error, mark that part as your answer
`("An unit an abstract idea")/a` `("defined either by reference to a randomly chosen")/b` `("material standard or to a natural phenomenon")/c` `("No error")/d`

Explanation

(a) Use .A unit. in place of .An unit.