Step 1: Understand the role of PCl5.
Phosphorus pentachloride, $PCl_5$, is a strong dehydrating and chlorinating agent. When it meets phosphoric acid, it pulls out water and joins the acid molecules together.
Step 2: Recognise the condensation.
Removing water from two acid units links them through an oxygen bridge. This joining of acid molecules is called condensation, and it makes a larger oxoacid.
Step 3: Identify the product.
By this dehydration and condensation, the oxoacid formed in this reaction is taken as \[ H_4P_2O_5, \] known as pyrophosphorous acid.
Step 4: Check the formula against options.
Among the choices, only $H_4P_2O_5$ matches the product formed here. The others ($H_4P_2O_6$, $H_4P_2O_7$, $H_3PO_3$) are different oxoacids.
Step 5: Note why others differ.
$H_4P_2O_7$ is pyrophosphoric acid and $H_3PO_3$ is phosphorous acid; they form under different conditions, not this one.
Step 6: State the answer.
So the oxoacid formed is \[ \boxed{H_4P_2O_5} \]