Concept:Wave front is the line representing points on a wave that vibrate in phase.Explanation:As a wave advances, particles along a line perpendicular to the direction of travel may have the same displacement and velocity.Such particles are said to be in the same phase.The line joining these particles is called the wave front.Wavelength is the distance between two successive identical points on the wave.Wave crest and trough are the highest and lowest points, not lines of equal phase.Answer:D. wave front.