5. $[x]$ represents taking the integer part of the number $x$, for example $\left[\frac{15}{4}\right]=3$,
$$
y=4\left(\frac{x+[u]}{4}-\left[\frac{x+[u]}{4}\right]\right),
$$

and when
$$
\begin{array}{lll}
x=1,8,11,14 \text { then } & y=1 ; \\
x=2,5,12,15 \text { then } & y=2 ; \\
x=3,6,9,16 \text { then } & y=3 ; \\
x=4,7,10,13 & \\
x=0 .
\end{array}
$$

then the expression for $u$ is
(A) $\frac{x+2}{4}$; (B) $\frac{x+1}{4} ;$ (C) $\frac{x}{4}$,
(D) $\frac{x-1}{4}$.
Answer: (D).