The graph is plotting cumulative rainfall and so in $24$ hrs $300$ mm of rainfall was received.
$300$ mm means the rainfall received in $1$ square meter is $300$ liters.
So, in $50\;m^2$ the amount of rainfall received is $300\times 50 = 15,000 \; l$
$50\%$ of this is harvested in tank. So,
Tank capacity $ = 0.5 \times 15,000\;l = 7,500\;l$
Option C.