Option 2 - "They were acquainted with the use of iron" - is the correct answer. The Rig-Vedic Aryans were indeed acquainted with horses, chariots, the use of bronze (option 1). The cow was also highly important to them and considered a form of wealth (option 3). However, they were not acquainted with the use of iron at this time. Iron was introduced to the Indian subcontinent much later, during the later Vedic age (around 1000 BCE). The technology of iron production and usage developed gradually over time.

It is worth noting that while the Rig-Vedic Aryans were acquainted with the use of copper (option 4) and may have used it for various purposes, the statement regarding the modern ploughshare is not accurate. The use of the modern ploughshare was not present in ancient times and is a later development.

In summary, the Rig-Vedic Aryans were not acquainted with the use of iron (option 2), but they were familiar with horses, chariots, bronze, copper, and valued cows as a form of wealth.