A screw thread specified by M $20\times 2.5C$ as per B/S thread system means
(a) Metric thread of 20 mm nominal diameter and 2.5 mm pitch having coarse tolerance
(b) Metric thread of 20 mm root diameter and 2.5 mm pitch having coarse tolerance
(c) Metric thread of line class having 20 mm root diameter and 2.5 mm pitch
(d) Metric thread of 20 mm shank diameter and 2.5 mm thread depth with coarse tolerance