Question

Last electron acceptor of $PSI$ in cyclic photo-phosphorylation is

Answer 1

During cyclic photo-phosphorylation, only $PSI$ is involved but not $PSII$. The reaction centre of $PSI$ absorbs light of wavelength $700nm$ and releases electrons which pass through the electron transport system and plastocyanin being the last electron acceptor of $PSI$.

This way the electrons are cycled back to the reaction centre. It does not require an external electron donor.