The correct option is
A Chair - Concrete Noun
Abstract nouns
refer to things that cannot be perceived through our senses
(touch, see, taste, smell, hear). They are emotions, activities or states of being.
For example, 'happiness', intelligence', etc.
Concrete nouns are the opposite of abstract: they can be perceived through the senses.
Option A: In the given sentence, the noun 'chair' is concrete because it can be seen, touched, etc. : "for him to sit on" tells that the chair can be perceived through touch. Hence option A is correct.
Options B and C: 'Him' is a pronoun and not a noun. Pronouns refer to nouns that have been mentioned before, but they themselves aren't nouns.
Option D: 'Comfortable' is an adjective and not a noun. Adjectives describe or give more information on nouns. Here, the adjective 'comfortable' describes the noun 'chair'.
Hence these options are wrong.