Box 1b (date acquired) is the date of your mother's death.
The basis of the stock is equal to its closing price on the date your mother died. If you know the number of shares (is that what you meant by "amount"?), then multiply the number of shares times the price on her date of death, and you have the amount for Box 1e. You can find the share price for any date by typing the ticker symbol into the internet and searching.
If you don't know the number of shares, you can calculate them. Find the share price on the date in box 1c. Divide proceeds by share price, and you will have the number of shares.
Inherited shares are always long-term. Rules about reporting purchase price changed in 2011; "covered" means that the purchase occurred after this regulation took effect. These shares would be Sales category E: Long term noncovered.
