Concept:
Mouldboard ploughs are classified according to size, duty and power source.
Step 1: Animal drawn ploughs are generally lighter than tractor drawn ploughs.
Step 2: A light duty animal drawn mouldboard plough is used for lighter field operations.
Step 3: The size of such a plough is expressed in millimetres.
Step 4: The size of a light duty animal drawn mouldboard plough is:
\[
100\,mm
\]
Step 5: Therefore, the correct answer is \(100\,mm\).
\[
\boxed{100\,mm}
\]