疲れているなら、少し休ん___どうですか。
Q. Which form completes the suggestion 'why don't you rest?'
Translation
If you're tired, why don't you rest a little?
Explanation
〜たらどうですか is a suggestion pattern: 休む→休んだら→休んだらどうですか. For 休む (mu-ending) the tara-form voices to んだら.