Step 1: Concept
The orthocentre is the point of intersection of the altitudes of a triangle.
Step 2: Analysis
The triangle is formed by $x=0$ (y-axis), $y=0$ (x-axis), and $x+y=1$. This is a right-angled triangle.
Step 3: Conclusion
In a right-angled triangle, the orthocentre is the vertex containing the right angle. Here, that is the origin $(0, 0)$.
Final Answer: (A)