In addition to studying finches, charles darwin closely studied a group of land turtles that had become physically different because of

a) inbreeding
b) bioengineering
c) artificial selection
d) environmental pressures.

The correct answer is-  D) environmental pressures.

When Charle's Darwin studied different species of finches on Galapagos islands, he proposed an evolutionary theory, called natural selection.

According to this theory, organisms that can better adapt in their environmental conditions, survive and increase their number through reproduction whereas other organisms that can not adapt are eliminated from the population. In due course of generations, species evolve and ultimately convert into another species.

In the similar manner, when he studied a group of land turtles, he found that turtles became physically different ( such as difference in the shape of tortoise like saddle back and dome shaped tortoise) because of environmental pressures.