Step 1: Examine the Assertion. Pure ethanoic acid ($CH_3COOH$) is indeed called GLACIAL acetic acid. Assertion is TRUE.

Step 2: Examine the Reason. The melting point of pure ethanoic acid is 290 K (about $17^{\circ}C$), which is above the normal winter temperature in cold climates. So in winter it freezes into an ice-like (glacier-like) solid. Reason is TRUE.

Step 3: Is R the explanation of A? Yes - it is precisely because it freezes to an ice-like solid in winter (due to its low melting point of 290 K) that pure ethanoic acid got the name 'glacial' acetic acid.

Hence the correct answer is (A).