The correct answer is: B. converts an input to a binary number.
An encoder is a device that converts information from one form to another. In the context of computers, an encoder converts an input signal, such as a keystroke or a mouse click, into a binary number. This binary number can then be used to represent the input in a computer program.
Option A is incorrect because an encoder does not send telegraph signals. Telegraph signals are sent using a device called a telegraph key.
Option C is incorrect because an encoder converts an input to a binary number, not a decimal number.
Option D is incorrect because an encoder does not have to have only four keys. Some encoders have more than four keys, and some have no keys at all.
Option E is the correct answer because it is the only option that is not contradicted by the definition of an encoder.