Naor Pinkas OT - senders security; reduction to DDH
Hint

Due to DDH we have that $(g^a, g^b, g^{a\cdot b}, g^c)$ and $(g^a, g^b, g^{c'}, g^c)$ are indistinguishable (where $c'$ is uniformly random and independent of $c$). 
By the same reason, $(g^a, g^b, g^c, g^{a\cdot b})$ and $(g^a, g^b, g^{c}, g^{c'})$ are indistinguishable.

Can you see how to put things together?