What's the correct orientation for a plug in this outlet? What are line, which are neutral, which is ground?
It's from India.
Sockets that can be used for both grounded and non-grounded 6A plugs are quite common in India. Other variants allow using grounded 6A and 16A plugs. Some examples are shown on the Digital Museum of Plugs and Sockets page http://fam-oud.nl/~plugsocket/OldBritish1.html (see images 8-13).
It seems likely that the images (a) and (b) show a socket that is designed for grounded (3-pin) and non-grounded (2-pin) 6 Amp / 240 Volt plugs. The lower two slots of the socket are Neutral (left) and Live/Hot (right). Together with the center top slot for grounding, they are meant for a 3-pin plug. The top two slots, at left and right of the larger ground slot are meant for a 2-pin plug.
The spacing of the L and N-pin of a a grounded 6A plug is 18.5 mm, whereas the spacing of the two pins of a non-grounded 6A plug is 16.6 mm. Unfortunately the photo is too vague to see whether the spacing of the top two slots is about 2 mm smaller than the spacing of the lower two slots.
The tolerances of Indian plugs and sockets are often quite large. It is often not too difficult to insert a 2-pin plug in the L and N slots meant for a 3-pin plug, which means that you can use both the top two slots (correctly) and the lower two. Using the lower two for a 2-pin plug is not really safe, because it is probably not possible to insert the plug completely. Note that in both cases the plug is inserted in a horizontal position.
The possibility two insert a plug in vertical position is remarkable. To my opinion it is a design fault. Usually the two slots at the left side are both connected to neutral and both slots at right to live/hot*. Insertion of a plug in vertical position is useless since both pins are connected to the same wire, either neutral or live. In both cases there is no potential difference.
Note that a connection to two hot poles may look comparable to split phase wiring which is commonly used in the U.S., but in the Indian situation the two live poles are identical = in phase, rather out of phase for split phase wiring.
The photo is too blurry for me to be certain, but it doesn't matter which way. That outlet and plug are not polarized and not grounded. Two opposite corners of the outlet are hot, and the other two are neutral. So you can plug in two plugs next to each other in either the horizontal or vertical position.
Why is it designed this way? Because the other option is to plug in one big plug that does have a ground connector, but can also draw about twice as many amps.
While the call may be rare for an electrical plug expert: there is just such a person who runs The Digital Museum Of Plugs and Sockets. Head over there for any deep plug research.