Explanation:

An analog computer is a type of computer that represents data as continuously varying physical quantities, such as voltage, pressure, or mechanical motion. It operates on continuous data, which can be seen as 'always changing' or continuously variable, directly modeling the problem using physical phenomena. Option (a) 'A machine which works on data which is always changea' (likely a typo for 'changing' or 'analog') accurately describes this characteristic. Option (b) 'An arithmetic high level language' describes a type of programming language, not a computer type. Option (c) 'Communicate at low level' describes a characteristic of machine-level communication, not the fundamental nature of an analog computer. Therefore, the most fitting description is that an analog computer handles continuously varying data ($ \text{a} $).