Step 1: Recall the IS 456:2000 provision.
According to IS 456:2000, the spacing of vertical stirrups in beams shall not exceed
\[
0.75d
\]
or
\[
300\,\mathrm{mm},
\]
whichever is less.
Step 2: Identify the correct option.
Hence,
\[
\boxed{0.75d\ \text{or}\ 300\,\mathrm{mm}}
\]
is the correct answer.
Therefore,
\[
\boxed{(D)}
\]
is the correct answer.