Solution:
The sentence indicates that there are other precious metal as well. Gold is one of them. 'A' article will be used.

Solution:
Here, 'orange' is a general fruit and therefore indefinite article will be used. Word after the blank starts with vowel sound. 'An' article will be used.

Solution:
Use 'an' before words such as "hour" which sound like they start with a vowel even if the first letter is a consonant. Also use 'an' before letters and numbers which sound like they begin with a vowel, such as "F" or "8".

Solution:
Indefinite articles 'a' or 'an' are used when the noun is not specific or the noun is common. 'Onion' is common noun and begins with vowel sound, thus takes 'an' article.

Solution:
Option C is correct as 'The' is used with words with superlatives. 'Largest' is superlative.

Solution:
No article is used before abstract/uncountable nouns. 'Time' is an abstract noun as used in this sentence.

Solution:
Definite article is used with positions, directions, numbers etc. Hence, 'last' will follow 'the' article.