On September 22, 2023, Astor Corporation invested $500,000 in short-term available-for-sale marketable securities. The market value of this investment was $550,000 at December 31, 2023, but had slipped to $525,000 by December 31, 2024. Assuming Astor does not sell this investment, the mark-to-market adjustment necessary at December 31, 2024, includes: Select one: O a. A $525,000 debit to Investments in Marketable Securities. O b. A $25,000 debit to Investments in Marketable Securities. c. A $25,000 credit to unrealized gain/loss on Investments. O d. A $25,000 debit to unrealized gain/loss on Investments.