First, second, and last are correct statements.
For accuracy and reliability of any experiment, a number of observations must be as large as possible. It reduces the effect of any observation or any event which might have happened by chance or error.
Thus, finches were better to record observations as they were available in large number as compared to the tortoise.
In addition, the life cycle of birds is smaller as compared to the tortoise which enabled the scientists to observe more generations.
Lastly, the birds were easy to capture and track as compared to the tortoise. Tortoise being an amphibian could live on land as well as in water which made it difficult to track them.