B

Step 1: Understanding the electromagnetic spectrum. The electromagnetic spectrum is divided into different regions based on the wavelength and frequency of electromagnetic waves. The main regions include radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
Step 2: Location of X-rays in the spectrum. X-rays have wavelengths ranging from about 0.01 to 10 nanometers, which is shorter than visible light (approximately 400 nm to 700 nm) but longer than gamma rays (typically less than 0.01 nm).
Step 3: Analyzing the options:
- Option A: Short radiowave and visible region - Incorrect, as X-rays are shorter than visible light.
- Option B: Visible and ultraviolet region - Correct, as X-rays lie between the visible light and ultraviolet region in the spectrum.
- Option C: Gamma rays and ultraviolet region - Incorrect since X-rays have longer wavelengths than gamma rays.
- Option D: Short radiowave and long radiowave - Incorrect, as this describes radio waves.
Therefore, the correct answer is B.