The correct answer is: B. 25Mhz
The 486DX2 processor is a second-generation x86 microprocessor from Intel, released in 1992. It is a clock-doubled version of the 486DX, running at twice the internal clock speed. The external bus speed of the 486DX2 is 25 MHz, which is half of the internal clock speed. This is because the 486DX2 uses a technique called clock doubling to achieve its higher clock speed. Clock doubling works by running the internal logic of the processor at twice the speed of the external bus. This allows the processor to execute instructions more quickly, but it also increases the amount of heat that the processor generates.
The other options are incorrect because they do not match the actual clock speed of the 486DX2 processor. Option A is incorrect because it is the internal clock speed of the 486DX, not the 486DX2. Option C is incorrect because it is the clock speed of the 486SX processor, which is a lower-end version of the 486DX. Option D is incorrect because it is the clock speed of the Pentium processor, which is a third-generation x86 microprocessor. Option E is incorrect because it is not a valid option.