Neither is correct. Your cost basis is the FMV of the stock on the day the person you inherited it from, passed away. Typically, you use the day's closing price.

@danielpelc 

You take the average of the high and low that the stock traded for on the date of death. 

Example: If the high for the day was $25 and low for the day was $24, your basis would be $24.50.

If the decedent (the person from whom you inherited the stock) died on a weekend, you would average the high and low for both Friday and Monday and then average those two prices.

My grandmother never owned the stock sold. She put cash into a trust for us. The trustee had the cash invested in stocks after her death. She died in 2008, stock was invested in 2015, sold in 2020. So it wasn't stock given by grandma but rather stock distributed in-kind. How does that affect it?

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@danielpelc wrote:

She died in 2008, stock was invested in 2015, sold in 2020. So it wasn't stock given by grandma but rather stock distributed in-kind. How does that affect it?

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Then you would take the trust's basis in the stock (i.e., the price the trust paid for the stock).

Similar question here. So the cost basis has not change upon distribution to beneficiaries? I’d understood that the gains between DOD and distribution were to be paid by the trust, not the beneficiary.

The basis does not change. 

Either the trust pays the tax on any gains and then distributed corpus or the trust passes through the gains to the beneficiaries and they pay tax on the gains.