Retirement tax questions

It seems you entered $25,000 instead of $10,000 on the "Total RMD for all other IRAs" screen (enter the RMD from your other IRAs where you didn't take a distribution from). This should be the $10,000 RMD for IRA-b where you didn't take a distribution.
 

1. Click on "Search" on the top right and type “1099-R”
2. Click on “Jump to 1099-R” and enter your 1099-R for IRA-a
3. On the "Required Minimum Distribution" screen enter RMD for this account $20,000
4. On the "How much of This Distribution Applied to your December 31, 2024 RMD" select select "The entire distribution applied…"  
5. Answer the "Transfer to Charity" screen select "Part was transferred to charity" 
6. Amount transferred to charity enter $30,000
7. On the "Your 1099-R Entries" screen add the 1099-R for IRA-c
8. On the "Required Minimum Distribution" screen enter RMD for this account $0
9. Answer the "Transfer to Charity" screen select "None was transferred to charity" 
10. On the “What Did You Do With The Money” and choose “I moved it to another retirement account”
11. Then choose “I did a combination of rolling over, converting, or cashing out money.” and enter the converted amount next to "Amount converted to a Roth IRA account" (S15,000)
12. On the "Your 1099-R Entries" screen click "Continue"
13. On the "Did You Miss a Required Distribution From Any Retirement Accounts?" screen select "An IRA..."
14. On the "Total RMD for all other IRAs" screen enter the RMD from your other IRAs where you didn't take a distribution from ($10,000 RMD for IRA-b).

You are not required to file Form 5329 since you met all of your RMD. TurboTax will enter the information in the "Additional Retirement Account Smartsheet" to check that you met the RMD. You can see this in TurboTax Desktop when you switch to form mode and look at Form 5329 but this will not be sent to the IRS.

When I enter it with the steps above I get this and it show the RMD for 2024 was met and Form 5329 will not be filled out and not sent to the IRS.

Note since you indicated IRA-c doesn't have a RMD it is not included here and you do not get the "How much of This Distribution Applied to your December 31, 2024 RMD" screen to indicate that you used this distribution towards the RMD. But the distribution from IRA-a covered all of your RMD for 2024 so this still shows you took all your RMD. And as mentioned before the Smartsheet is only for your records and does not get sent to the IRS.