The correct option is A The
The given sentence "____ highest mountain peak of the world" is in the superlative degree of comparison. 'Superlative' means 'of the highest quality or degree'.
In such a sentence, the article 'the' is placed before the superlative adjective 'highest'. This is because 'the' indicates a specific noun; here it modifies a noun 'mountain peak' which is made specific because it is the 'highest peak of the world'.
Hence, the blank will be filled with 'the' and option A is correct.
Options B and C: 'A' and 'an' are indefinite articles; they refer to nouns that are non-specific. For example, 'a man', 'an animal'. But, the given sentence mentions a specific noun 'highest mountain peak in the Himalayas'. Hence B and C are incorrect.
Option D is incorrect because the noun requires an article. (We do not use an article when referring to unique names or places.)