The correct option is B too young
Let’s examine the sentence: “My son is seventeen years old. He is ____ to get married.” There are two related sentences here. The first sentence states the age of the son and the second sentence states the fact of his getting married. Using common sense, we can understand that seventeen is not the age of getting married and the son is young for getting married. This fact rules out Options A and B that state that the son is old enough to be married. Options C and D use the adjective ‘young.’ Now, the sentence requires an adverb of degree with ‘young’ that can convey the sense that at seventeen, the son is extremely young to get married. Adverbs of degree tell us about the intensity of something. Adverbs of degree are usually placed before the adjective, adverb, or verb that they modify, although there are some exceptions. The sentence requires an adverb of degree that can emphasize strongly the extremity of youth. The adverb ‘too’ conveys this sense as it means ‘excessively’ or ‘to a great extent.’ The adverb ‘enough’ does not convey this sense as its meaning convey the sense of ‘to the necessary degree.’ So, Option D containing ‘enough’ would convey the meaning that the son is of the age to get married. This is not what the sentence wants to convey. Hence, option D cannot be used. So, Option C is the correct answer as it conveys the intended meaning of the sentence.