Question

The acid which is a solid

Answer 1

Solid acids are acids that do not dissolve in the reaction medium. They are often used inheterogeneous catalysts.

ExamplesEdit

Examples include oxides, which function asLewis acids including silico-aluminates (zeolites, alumina, silico-alumino-phosphate), and sulfated zirconia. Many transition metaloxides are acidic, including titania, zirconia, and niobia.[1] Such acids are used in cracking. Many solid Brønsted acids are also employed industrially, including sulfonated polystyrene, solid phosphoric acid, niobic acid, and heteropolyoxometallates.[2]

Answer 2

I think its sulphur.
Sulphur is normally solid but sulphuric acid also exists.