Concept:
Seasonal employment (or seasonal unemployment) occurs when employment opportunities are available only during particular seasons of the year. It is common in agriculture where labour demand rises during sowing and harvesting seasons and falls during the off-season.
Many migrant workers move from urban informal sectors to rural areas during peak agricultural seasons, causing periodic changes in labour supply.
Step 1: Examine each statement.
• [(A)] Unlimited supply of labour is associated with surplus labour or disguised unemployment, not seasonal employment. Incorrect.
• [(B)] Agricultural labour demand rises during sowing and harvesting seasons. Correct.
• [(C)] During peak agricultural seasons many migrant workers return to villages, reducing the supply of unorganised labour in urban areas. Correct.
• [(D)] Involuntary unemployment is a different concept and is not the definition of seasonal employment. Incorrect.
Thus,
\[
\boxed{B \text{ and } C}
\]
Hence,
\[
\boxed{\text{Correct Answer}=(C)}
\]