What is ubiquinol?

In the electron transport system in the inner mitochondrial membrane,complex-$I$ and $IV$ are respectively:

Which process is common to both respiration and photosynthesis?

Match the following columns regarding the Electron Transport System $(ETS)$:

Column-$I$ ($ETS$ Complex)	Column-$II$ (Name of Complex)
$P$. Complex-$I$	$I$. Cytochrome $c$ oxidase complex
$Q$. Complex-$II$	$II$. Cytochrome $bc_1$ complex
$R$. Complex-$III$	$III$. $ATP$ synthase complex
$S$. Complex-$IV$	$IV$. $FADH_2$ dehydrogenase complex
$T$. Complex-$V$	$V$. $NADH$ dehydrogenase complex

Given below are two statements.
Statement-$I$: Cytochrome $c$ is a small, copper-containing protein loosely attached to the inner membrane.
Statement-$II$: Cytochrome $c$ acts as a mobile electron carrier transferring electrons between complex $I$, $II$, and $IV$.
In the light of the above statements, choose the correct answer from the options given below.

The synthesis of $ATP$ in photosynthesis and respiration is essentially an oxidation-reduction process involving the removal of energy from: