**Rule: Use 'the' when a noun is made specific by a defining relative clause.** The noun 'boy' is not just any boy; it is the specific one 'who won the race'. This clause defines the boy, making him definite. Therefore, 'the' is required. Option (a) would be incorrect as the boy is specified. Option (b) is wrong by sound. Option (d) is incorrect.

**Rule: In exclamatory sentences beginning with 'What' followed by a singular countable noun, 'a' or 'an' is used.** 'Coincidence' is a singular countable noun, and the adjective 'strange' begins with a consonant sound. Therefore, 'a' is correct. Option (b) is wrong by sound. Option (c) would be incorrect for a general exclamation. Option (d) is ungrammatical.

**Rule: 'The' is used before a unique post or designation.** 'The post of' is a phrase that introduces a specific, unique position. 'Deputy Commissioner' is a singular, specific role in this context, so the definite article 'the' is required before 'post'. Option (a) and (c) are indefinite. Option (d) is incorrect; 'the post' is a fixed phrase.

**Rule: 'The' + adjective is used to represent a whole class of people.** 'The needy' refers to all people who are in need, as a group. The structure is parallel to 'the homeless' which is also mentioned. Using 'the' before the adjective turns it into a plural noun. Options (a), (b), and (d) are incorrect.

**Rule: The choice of 'a' vs. 'an' depends on the sound, not the spelling.** The word 'one' starts with the vowel 'o' but is pronounced with a 'w' sound, which is a consonant sound. Therefore, the article 'a' must be used before it. Option (b) is a common error. Option (c) would imply a specific coin. Option (d) is incorrect for a singular countable noun.

**Rule: Material nouns used in a general sense do not take an article.** 'Silver' is a material noun being discussed as a substance in general. Therefore, no article is required. If the sentence referred to specific silver (e.g., 'The silver in this necklace is pure'), 'the' would be used. Options (a), (b), and (c) are incorrect.

**Rule: No article is used in phrases denoting the mode of transport like 'by sea', 'by air', 'by road', 'by train'.** These are fixed expressions where the noun represents a general concept of travel, not a specific sea or road. Therefore, no article is the correct choice. Options (a), (b), and (c) are all incorrect.

**Rule: 'In a hurry' is a fixed idiomatic expression.** The noun 'hurry' in this phrase is always preceded by the indefinite article 'a'. 'Hurry' begins with a consonant sound. Option (b) is wrong by sound. Option (c) and (d) are incorrect for this fixed phrase.

**Rule: 'A little' means 'some', which fits the meaning of this proverb.** This is a famous saying. 'A little knowledge' means 'some amount of knowledge'. The proverb warns that having a small amount of knowledge can be dangerous. 'Little knowledge' (without 'a') would mean 'almost no knowledge', which doesn't fit the proverb's meaning. 'The little' would refer to a specific small amount of knowledge. 'An' is incorrect by sound.

**Rule: 'The' is used to refer to a specific, unique place or position.** 'The summit' refers to the single, specific peak of the mountain they were climbing. The context implies a definite location. The first part of the sentence also uses 'the' with the ordinal 'first'. Option (a) or (b) would wrongly suggest there were multiple summits to choose from. Option (d) is incorrect.