The correct answer is - D) Trade is limited because of physical barriers on land and water.
Looking strictly from a geographical point of view, the two Asian giants, China and India, have troubles with the trade because of the geographical obstacles, both on land and water.
On land the big problem are the Himalayas and the Tibetan Plateau, while on water its the location of Indochina as it is right on the way of these two countries, thus it has to be surrounded in order to reach one another.
Now that has been a huge problem until relatively recently but nowadays, with the modern technology and improvement in the transportation, that is no longer a big issue, and the trade between the two is going relatively well.