If the fair value of the portfolio of trading securities was less than the cost, then an adjustment would

a$.$ credit Unrealized Loss on Trading Investments and debit Valuation Allowance for Trading Investments for the difference.

b$.$ not be recorded until the trading security was sold.

c$.$ debit Operating Expense and credit Valuation Allowance for Trading Investments.

d$.$ debit Unrealized Loss on Trading Investments and credit Valuation Allowance for Trading Investments for the difference.