Those who knew him will be sorry to hear that he was killed last week,on Western Front.

Explanation

"Western Front" is missing the definite article "the." It should be "the Western Front." There is an error here.The error in the sentence is the omission of the definite article "the" before "Western Front." The correct phrase should be "on the Western Front." Thus, the correct option to mark the error is D) Western Front.