The greatest perfect square number of 6 digits is:

• 999001

• 998001

• 998009

• 998009

B.

998001

Think of the smallest perfect square with 7 digits. It's 1,000,000 which is 1000 squared. Now try smaller numbers squared.
 999 squared is 998001. There's no integer between 999 and 1000, so 998001 is the largest perfect square with 6 digits.