Answer:
The length of the diagonal AC=20 cm.
Step-by-step explanation:
From ΔOBP and ΔABC, we have
( O and P are mid points)
∠B=∠B(Common)
Therefore, by SAS rule,
ΔOBP is similar toΔABC
Thus, 
⇒
⇒
⇒
⇒
Thus, the length of the diagonal AC=20 cm.
The line passing through points
and
is given by

Plug your values to get

Multiply both sides by 8 to get

I think it is choice 3 (c)