Concept:
The order of a differential equation is equal to the highest order derivative present in it. For a family of curves, the order is generally equal to the number of arbitrary constants.
Step 1: Since the centres are on the \(x\)-axis, let the centre be:
\[
(a,0)
\]
Step 2: Since the circle passes through the origin, its radius is:
\[
r=a
\]
Step 3: Equation of the circle is:
\[
(x-a)^2+y^2=a^2
\]
Expanding:
\[
x^2-2ax+a^2+y^2=a^2
\]
\[
x^2+y^2-2ax=0
\]
Step 4: This family contains only one arbitrary constant \(a\).
So we need to differentiate once to eliminate \(a\).
Step 5: Differentiate:
\[
x^2+y^2-2ax=0
\]
\[
2x+2y\frac{dy}{dx}-2a=0
\]
The highest derivative present is:
\[
\frac{dy}{dx}
\]
So the order is:
\[
1
\]
Therefore,
\[
\boxed{1}
\]