CBSE Class 12 · 2006 · SemiconductorsShort Answer

The series diode conducts only when it is forward biased.
When the input is $+5\mathrm{V}$ the diode is forward biased, so almost the full $+5\mathrm{V}$ appears across $R$.
When the input is $-5\mathrm{V}$ the diode is reverse biased, no current flows and the output across $R$ is zero.
Hence the output consists of positive half-pulses only (half-wave rectified).
$\boxed{\text{Positive halves pass; negative halves are blocked.}}$