Fill in the blanks with the most appropriate option:-

It has been a long time………. I visited Agra.

Since 

The most appropriate option to fill in the blanks is since.

The sentence "It has been a long time since I visited Agra" is in the present perfect tense, which is used to talk about an action that started in the past and is still happening now. The other options are not as appropriate.

• For is used to talk about a period of time. This does not make sense in the context of the sentence, as the speaker is not talking about a specific period of time, but the fact that it has been a long time since they visited Agra.
• From is used to talk about a starting point. This does not make sense in the context of the sentence, as the speaker is not talking about a specific starting point for their visit to Agra, but the fact that it has been a long time since they visited.
• Till is a British English word that is used to talk about a point in time in the past. This does not make sense in the context of the sentence, as the speaker is not talking about a specific point in time in the past, but the fact that it has been a long time since they visited Agra.