Muy 1099-B contains a sales section title for "unknown gains or lossess" which reports proceeds, but has "N/A" for cost or basis. How do I enter this in the Sales section title selection which does not contain this as a choice?

You can't report the sales as undetermined on your tax return. "Undetermined" on the 1099-B means that the brokerage doesn't know whether it's short-term or long-term. You have to determine that from your own records of when and how you acquired the investments. Determine the acquisition date as accurately as you reasonably can, but the exact date is not critical as long as you correctly determine whether each lot is short-term or long-term. Then, in TurboTax, you will select the sale category as either "Short term basis not reported" or "Long term basis not reported."

If the 1099-B lists the sales as undetermined, it probably does not show the acquisition date and basis. You can enter "Various" as the acquisition date as long as you correctly categorize the sale as short-term or long-term. If you inherited the investment you can enter "Inherited" as the acquisition date. An inherited investment is always considered long-term, regardless of how long you or the deceased actually owned it. If you received the investment as a gift, your acquisition date is the date that the donor acquired it.

You also have to provide the basis from your own records. If you purchased the investment yourself the basis is what you paid for it. If you inherited it, the basis is the market value on the date of death of the person you inherited it from. If you received it as a gift, your basis is the donor's basis.

For investments that you purchased, if you have a reasonable estimate of the purchase date, there are web sites where you can look up historical prices of stocks and mutual funds.

Ask the broker if they have any information about the acquisition date and purchase price. Sometimes they have the information, even though they don't put it on the 1099-B. But they are not likely to have any information about investments that you inherited or received as a gift.