Answer: Option C
Let the total work be 1 unit.
Anu can do the work alone in 12 days, so the amount of work she can do in one day is 1/12.
Radha can do the work alone in 18 days, so the amount of work she can do in one day is 1/18.
Let Ratna's efficiency be x. Then, the amount of work she can do in one day is 1/x.
Together, in 4 days, they completed the whole work, so their combined efficiency is 1/4.
From the given information, we can write the following equation:(1/12 + 1/18 + 1/x) x 4 = 1
Solving this equation, we get x = 36.
Therefore, Ratna can do 1/36th of the work in one day.
Now, we can find the amount of work done by each person in 4 days:Anu can do (1/12) x 4 = 1/3 of the workRadha can do (1/18) x 4 = 2/9 of the workRatna can do (1/36) x 4 = 1/9 of the work
Hence, their shares in the total amount of money can be calculated as follows:Anu's share = (1/3) / (1/3 + 2/9 + 1/9) * 1800 = Rs. 600
Therefore, the correct option is (C) Rs. 600.
Key Points:

• Let total work be 1 unit
• Use formula: Efficiency = amount of work / time
• Let Ratna's efficiency be x
• Equate the combined efficiency with total work
• Solve for x to get Ratna's efficiency
• Calculate the amount of work done by each person in 4 days
• Calculate each person's share in the total amount of money
• Anu's share = (her work / total work) * total money.