Explanation:

A computer is fundamentally an electronic device that processes data according to a set of instructions. Option (a) 'Electronic machine' accurately describes this core nature, as computers rely on electronic circuits to perform operations. Option (b) 'Power machine' is incorrect as it implies power generation or heavy mechanical work, which is not a computer's primary function. Option (c) 'Electric machine' is too broad, as many non-computing devices, like a toaster or a fan, are electric machines. Option (d) 'All of these' is incorrect because options (b) and (c) are not precise or accurate descriptions of a computer. Option (e) 'None of these' is incorrect because (a) is a correct and fundamental definition.