Final answer:
The mass of an electron is constant and remains 9.11 x 10^-31 kg, irrespective of its speed.
Step-by-step explanation:
The mass of an electron does not change regardless of its speed; it remains constant at a known value. The rest mass of an electron is 9.11 × 10^-31 kg, which is not affected by the velocity of the electron. Therefore, even if an electron is moving with a speed of 5.00 × 10^9 cm/sec, the mass remains 9.11 × 10^-31 kg (option a).