Step 1: Use the horizontal tension formula for a cable carrying UDL.
\[
H=\frac{wL^2}{8d},
\]
where
\[
w=6\,\mathrm{kN/m},\qquad
L=10\,\mathrm{m},\qquad
d=3\,\mathrm{m}.
\]
Step 2: Calculate the horizontal tension.
\[
H=\frac{6\times10^2}{8\times3}
=\frac{600}{24}
=25\,\mathrm{kN}.
\]
Hence,
\[
\boxed{25\,\mathrm{kN}}
\]
is the correct answer.