Step 1: Understanding the Concept:
We can check this figure by comparing units. The options are given in a mix of mm and cm, so first put them on the same footing.
Step 2: Key Formula or Approach:
Convert every option to millimeter: 40 mm stays 40 mm, 40 cm becomes 400 mm, 10 cm becomes 100 mm, and 200 mm stays 200 mm.
Step 3: Detailed Explanation:
Rice nurseries are raised in small, shallow beds kept flooded with a few centimeters of standing water for close to a month, so that seedlings establish well before pulling and transplanting.
Because the nursery keeps standing water continuously over this whole period, its water demand, once spread over the 1 ha of main field it will eventually supply seedlings for, is a moderate but real quantity, and Kung's cited figure for this is 200 mm.
This value sits between the too-low options (40 mm and 100 mm) and the too-high option (400 mm), and matches the standard textbook citation.
Step 4: Final Answer:
The nursery water requirement per hectare of main field, per Kung (1971), is 200 mm, option 4.
\[ \boxed{200 \text{ mm}} \]