If I received cash from a traditional IRA this year in a lesser amount than my usual RMD, which box do I check: "entire distribution was RMD" or "none of this was RMD"?

[DoninGA]

The required minimum distribution for any year is the account balance as of the end of the immediately preceding calendar year divided by a distribution period from the IRS’s “Uniform Lifetime Table.” A separate table is used if the sole beneficiary is the owner’s spouse who is ten or more years younger than the owner. 

For tax year 2020 an RMD was not required so you can check None was an RMD.

[MinhT1]

The RMDs have been waived for tax year 2020.

Please read this IRS document for more information.

In TurboTax, if you withdrew in 2020 less than your RMD, you can choose None of this withdrawal was a RMD. There is no penalty.

[fanfare]

the question only applies if you did or tried to do a Rollover.

otherwise it doesn't matter how you answer