Which of the following signs requires a driver to yield to oncoming vehicles?

Explanation

Sign 2.6 "Priority for oncoming traffic" obliges you to give way to oncoming vehicles: you may not enter the narrow section if that could hinder oncoming traffic. The square sign 2.7 does the opposite and gives you priority, 2.5 requires a stop before the stop line, and 1.21 warns of two-way traffic (Rules, signs 2.5, 2.6 and 2.7).