  For the first time, the nuclear effects in the reaction \nu p -> \mu^- p
\pi^+ on bound protons are investigated at E_\nu = 9 GeV and the effective
atomicweight A = 21 of the composite nuclear target using the data obtained
with SKAT bubble chamber. The observed nuclear effects are explained by the
Fermi-motion of the target proton and secondary elastic scattering of
neutrinoproduced hadrons on intranuclear neutrons. The probability of any
intranuclear interaction of neutrinoproduced hadrons is extracted: P_int =
0.40+-0.13, while the probability of interactions occurring only via elastic
scattering on intranuclear neutrons is estimated to be P_c = 0.14+-0.05. A
clear evidence of the \Delta^{++}(1232) and an indication on the
\Delta^{++}(1620) isobar states production are observed with the mean yields,
respectively, 0.57+-0.09 and 0.40+-0.26 (the latter being corrected for the p
\pi^+ decay fraction). The mean yield of \Delta^{++}(1232) is by 1.61+-0.25
times smaller than expected for the reaction on the hydrogen target, thus
indicating on its rather strong nuclear absorption.
