Step 1: Express \(\overrightarrow{AC}\) using vector addition.
We know that
\[
\overrightarrow{AC}
=
\overrightarrow{AB}+\overrightarrow{BC}
\]
Given,
\[
\overrightarrow{BC}=\lambda\overrightarrow{AD}
\]
Hence,
\[
\overrightarrow{AC}
=
\overrightarrow{AB}+\lambda\overrightarrow{AD}
\]
Step 2: Express \(\overrightarrow{BD}\).
Also,
\[
\overrightarrow{BD}
=
\overrightarrow{BA}+\overrightarrow{AD}
\]
Since
\[
\overrightarrow{BA}=-\overrightarrow{AB},
\]
we get
\[
\overrightarrow{BD}
=
-\overrightarrow{AB}+\overrightarrow{AD}
\]
Step 3: Add \(\overrightarrow{AC}\) and \(\overrightarrow{BD}\).
Now,
\[
\vec{x}
=
\overrightarrow{AC}+\overrightarrow{BD}
\]
Substituting the obtained expressions,
\[
\vec{x}
=
(\overrightarrow{AB}+\lambda\overrightarrow{AD})
+
(-\overrightarrow{AB}+\overrightarrow{AD})
\]
Step 4: Simplify the expression.
Cancelling
\[
\overrightarrow{AB},
\]
we get
\[
\vec{x}
=
\lambda\overrightarrow{AD}+\overrightarrow{AD}
\]
\[
=
(\lambda+1)\overrightarrow{AD}
\]
Step 5: Compare with the given form.
Given,
\[
\vec{x}=p\overrightarrow{AD}
\]
Comparing,
\[
p=\lambda+1
\]
Step 6: Match with the options.
The obtained value is
\[
\lambda+1
\]
Step 7: Final conclusion.
Therefore,
\[
\boxed{\lambda+1}
\]