Final answer:
Mendeleev created a new row in his periodic table when the previous row was complete, as to group elements with similar properties together. This constituted the completion of one period and the beginning of a new one. The modern periodic table still retains this group and period structure.
Step-by-step explanation:
Dmitri Mendeleev created a new row in his periodic table when the previous row was complete. This means the correct answer is D: When the previous row was complete. Mendeleev arranged the elements in a table based on atomic masses and chemical properties. Elements with similar properties were placed in the same vertical columns, known as groups, and each row was referred to as a period. The period would be considered complete when no other elements with similar properties were left to be added to that row, thereby necessitating the start of a new row for the next group of elements with recurring properties.
The periodic table has undergone extensive developments since Mendeleev's original design. Mendeleev's table has gained widespread acceptance, especially after elements he predicted were discovered with properties matching his forecasts. The modern periodic table is now arranged by atomic numbers, but still retains the group and period organization that reflects similar properties in elements.