The problem states that $x$ km/h is equal to $y$ m/s. We need to find the expression for $y$ in terms of $x$. In other words, we need to convert km/h to m/s.

1. Problem Description

The problem states that $x$ km/h is equal to $y$ m/s. We need to find the expression for $y$ in terms of $x$. In other words, we need to convert km/h to m/s.

2. Solution Steps

First, let's convert km to meters. We know that 1 km = 1000 m.

Next, let's convert hours to seconds. We know that 1 hour = 60 minutes, and 1 minute = 60 seconds. Therefore, 1 hour = 60 * 60 = 3600 seconds.

Now we have:

$x \frac{km}{h} = x \frac{1000 m}{3600 s}$

Simplifying the fraction:

$x \frac{1000 m}{3600 s} = x \frac{10}{36} \frac{m}{s} = x \frac{5}{18} \frac{m}{s}$

Since $x$ km/h = $y$ m/s, we have:

$y = \frac{5}{18} x$

3. Final Answer

D. $\frac{5}{18}x$