Mountain ranges take the definite article 'the'. 'The Himalayas' refers to a specific range. Omitting the article would be ungrammatical. Thus, 'the' is required.

The adjective 'useful' begins with the consonant 'yoo' sound, so 'a' is correct. The observation is first mentioned and non-specific. 'An' would be phonetically inappropriate here. 'The' would require prior identification.

Although 'sugar' is uncountable, the situational context (the sugar on the counter) makes the reference definite. The definite article marks shared situational knowledge. No article would be too general. Indefinite articles do not pair with uncountable nouns in this sense.

The noun 'elephant' begins with a vowel sound, necessitating 'an'. The referent is introduced for the first time, favoring the indefinite article. 'A' would be phonetically incorrect. 'The' would imply prior mention of that specific calf.

In 'honorable', the initial 'h' is silent, yielding a vowel sound. Therefore, 'an' is correct. 'A honorable' would clash with English phonotactics. 'The' would suggest a previously specified decision, which is not the case.

Although 'uniform' begins with the letter 'u', it starts with the consonant 'yoo' sound, requiring 'a'. The item is new to the discourse, so an indefinite article is appropriate. 'An' would be phonetically incorrect. 'The' would imply a particular uniform already identified.

The phrase 'of the ancient temple' specifies which architecture is meant, making the noun definite. The definite article is therefore necessary. Without 'the', the referent would be ambiguous. Indefinite articles contradict the specificity introduced by the prepositional phrase.

The word 'espresso' begins with a vowel sound, so 'an' is the correct choice. The drink is mentioned for the first time, supporting the indefinite article. Using 'a' would violate the vowel-sound rule. 'The' would imply a previously specified espresso.

The determiner 'only' makes the noun uniquely identifiable, triggering the definite article. 'The only person' is the standard collocation. Indefinite articles do not co-occur with 'only' in this sense. Therefore, 'the' is correct.

Proper names of rivers take the definite article 'the'. 'The Amazon River' refers to a unique, specific geographical entity. Omitting the article or using an indefinite one would be ungrammatical. Hence, 'the' is required.