Myron's "Discobolus", also known as the "Discus Thrower", is a renowned piece from the classic Greek art period. It was originally crafted by the Greek sculptor Myron around 450 BCE. This sculpture represents an athlete in the moment of throwing a discus, capturing a perfect balance of movement and calmness typical of Greek art.

The original sculpture by Myron was created in bronze. Bronze was a widely used material for sculptures in ancient Greece due to its durability and versatility, allowing artists to capture fine details and dynamic poses. Many of these original bronze sculptures have not survived, but their forms are known through Roman copies often made in marble.

Let's review the options:

1. Cement: This is a modern construction material and was not used for classical sculptures of Myron's time.
2. Marble: Although many ancient Greek sculptures are seen in marble today, marble copies of "Discobolus" are of Roman origin. The original by Myron was not made in marble.
3. Bronze: Correct answer. The original "Discobolus" was crafted in bronze.
4. Sandstone: This material was not commonly used for Greek sculptures of this period.

Therefore, the correct answer is Bronze.