Parker marks sixths on a number line. He writes

5
6
just before 1.

What fraction does he write on the first mark to the right of 1?

A.
1
6
B.
7
7
C.
7
6
D.
4
6

If Parker marks sixths on a number line, then the distance between consecutive marks is $1/6$. Since Parker writes the numbers $5/6$ and $6/6$ just before 1, the mark to the right of 1 is $7/6$, which is $\boxed{\textbf{(C) }7/6}$.