I use an adjacent cell that's formatted in a big, bold, red font.

=if(a1<"","","<-- Please answer this question")

But why would you allow 3 answers in your data|validation list.

You could just use Yes and No, but it won't stop users from clearing the cell.
But it will stop them from choosing the blank.

Wanna Learn wrote:

Hello The data validation box has three answers, blank space, Yes, No)
The answer must be either yes or No but they must answer the question. What
do I do so that the question is answered and not skipped? thank you

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Dave Peterson