Every sentence begins with a capital letter.
The given sentence is a declarative sentence. Hence, it has to end with a full stop.
The subject of the sentence is the noun phrase, 'My brother'. It is described by the adjective clause, 'who is an athlete'. This adjective clause is non-essential. i.e it is not needed for the sentence to be complete. The sentence, 'My brother has won a silver medal in relay race' is complete in meaning without the aforesaid clause.
Such non-essential adjective clauses are separated from the rest of the sentence by commas. i.e. A comma is placed both before and after the non-essential adjective clause.
The first letters of adjectives and common nouns in the middle of the sentence should not be capitalised.
Option A: All the above-said punctuation rules are followed in this sentence. This option is correct.
Option B: The first letters of the adjective, 'silver' as well as the common nouns 'medal, 'relay' and race' are incorrectly capitalised. Hence this option is incorrect.
Option C: The sentence does not begin with a capital letter. The adjective clause, 'who is an athlete', is not correctly punctuated with commas before and after the clause. Further, in this option, the assertive sentence incorrectly ends with a question mark. Hence, this option is incorrect.
Option D: The adjective clause, 'who is an athlete', is not correctly punctuated with commas before and after the clause. Hence, this option is incorrect.