What does the Moody's Analytics model use as equivalent to holding a call option on the assets of the firm?

Question $5$ options:

$1)$

The value of equity in a firm.

$2)$

Total liabilities of a firm.

$3)$

Net income of a firm.

$4)$

Dividend yield of investments.

$5)$

Short$-$term debt liabilities of a firm.