Introduction:
The question asks to identify the correct sentence that describes the Tilak Express as the fastest train in India. Let's analyze each option to determine the correct answer.

Analysis of Options:

a) A, a
This option uses the indefinite articles "A" and "a" before the words "Tilak Express" and "fastest train," respectively. However, since the Tilak Express is a specific train, the definite article "the" should be used. Therefore, this option is incorrect.

b) The, the
This option uses the definite article "the" before both "Tilak Express" and "fastest train." Since the Tilak Express is a specific train, the use of "the" is appropriate. Moreover, since "fastest train" refers to a specific train as well, the use of "the" is correct in this context. Therefore, this option is correct.

c) A, the
This option uses the indefinite article "A" before "Tilak Express" and the definite article "the" before "fastest train." However, since the Tilak Express is a specific train, the definite article "the" should be used. Therefore, this option is incorrect.

d) A, an
This option uses the indefinite article "A" before "Tilak Express" and the indefinite article "an" before "fastest train." However, since the Tilak Express is a specific train, the definite article "the" should be used. Therefore, this option is incorrect.

Conclusion:
Based on the analysis, option b) "The, the" is the correct sentence that describes the Tilak Express as the fastest train in India. It uses the definite article "the" appropriately before both "Tilak Express" and "fastest train," indicating their specificity in this context.