  We report an experimental test of complementarity using clock-triggered
single-photon pulses emitted by an individual N-V color center in a diamond
nanocrystal. The single photons are sent into a Mach-Zehnder interferometer
with an output beamsplitter of adjustable reflection coefficient R. In
addition, the choice of introducing or removing this beamsplitter is random and
relativistically space-like separated from the entering of the photon inside
the interferometer, as required for the Wheeler's delayed-choice regime. Each
set value of R allows us to observe interference with visibility V and to
obtain incomplete which-path information characterized by the
distinguishability D. The measured values of V and D are found to obey the
complementarity relation V^2 + D^2 =< 1.
