Because you were married for some time in 2021, you may file a joint return for the year for you and your deceased spouse, provided you did not remarry before the end of 2021. If there is an executor or estate representative other than yourself, that representative must agree to the joint return.
Increase in value of property due to market conditions. When you sell appreciated property, you pay tax on the appreciation since the date of purchase. When you donate appreciated property held long term, you may generally deduct the appreciated value.