Strong RSA keys are at least ________ bits long.

• A. 100
• B. 256
• C. 512
• D. 1,024

Answer: (D)

RSA key length is about how hard it is to factor the modulus, the product of two large primes. Shorter moduli are easily factored with current methods, so the smaller options would not be considered strong. A 1024-bit modulus has historically been the minimum length viewed as providing a practical security margin, making it the best fit among the choices. Keep in mind that modern standards often require even larger keys, like 2048 bits or more, due to advances in factoring techniques and computing power.