Step 1:
Apurva refers to the unseen spiritual force generated by ritual actions.
Step 2:
This concept explains how Vedic rituals produce future results.
Step 3:
The doctrine of Apurva is mainly associated with:
\[
\boxed{\text{Purvamimamsa}}
\]
Step 4:
Therefore, the correct option is:
\[
\boxed{\text{(2) Purvamimamsa}}
\]