The segment that contains a grammatical error is Option C.

The error in this segment is the incorrect use of the indefinite article “a” before a word starting with a vowel sound. The correct phrase should be “as an essential”. The indefinite article “an” is used before words that begin with a vowel sound. 

Therefore, the corrected sentence would be: “In today’s motivational literature, failure is often celebrated as an essential stepping stone to success.” 

The other segments are grammatically correct.