Step 1: Meaning of the idiom.
The idiom “writing on the wall” originates from the Biblical story in the Book of Daniel, where mysterious writing foretold the fall of the Babylonian king.
In modern English, it means: a sign or warning that something bad is going to happen — essentially a foreboding.
Step 2: Eliminate distractors.
(A) Graffiti — literal writing on a wall, unrelated to idiomatic sense.
(B) Obvious truth — could be visible, but does not convey the sense of impending danger.
(D) Prediction — too broad; the idiom refers to a negative prediction.
\[
\boxed{\text{(C) Foreboding}}
\]