The correct option is
A We went to see the Tajmahal, not only alone, but also once more, along with friends.
The conjunction, 'Not only.. but also' is a correlative conjunction, which combines two grammatically equal items. This means that the words which follow both parts (i.e 'not only' as well as 'but also') should belong to the same parts of speech. Using different parts of the speech after 'not only' and 'but also' makes the sentence imbalanced and grammatically incorrect.
Option A: When a sentence begins with 'not only, it is followed by an auxiliary verb like 'did'/does'/'is' + subject. In this option, the sentence begins with 'Not Only'. However, this is not immediately followed by any auxiliary verb. Hence, this option is incorrect.
Option B: The phrase, 'not only' is followed by 'alone', which is an adverb. The phrase 'but also' is followed by 'once more, along with friends', which is also an adverbial phrase. Hence, this option is grammatically correct.
Option C: In this sentence, the phrases 'not only' and 'but also' are used together, which is grammatically incorrect.
Option D: The phrase, 'not only' is followed by 'to see the Tajmahal' -an infinitive phrase, which acts as an adverb. The phrase 'but also' is not used in this sentence. Hence, this option is grammatically incorrect.