The correct option is A thinner
A sentence is in the comparative degree when two nouns are compared to each other, one noun being higher in quality than the other. The comparison here is between two pieces of wood, where one is 'thinner' than the other. The adjective 'thin' forms a comparative by adding '-er' to it, 'thinner' hence option C is correct. Option A is incorrect because the comparative 'more' does not precede adjectives which have one or two syllables (at least in the given case). While converting a positive to a comparative, we add the suffix 'er'. When the positive adjective of one syllable ends in a single consonant (n) and is preceded by a short vowel (i), like in 'thin' then that consonant is doubled. Hence option B is incorrect. Option D is incorrect because the phrase is of the positive degree.