While looking for a singer for his party, Mr. Mehta met three girls named Teena, Shobha and Kavita who always gave two replies to any questions in which one is true and the other is false. Among all the three girls one is the truth teller, one is a liar, and one is an alternator. Mr. Mehta gets confused listening to them. Now, it is your task to find out who among the three girls is the singer and get rid of the confusion for Mr. Mehta.

Kavita
I am the singer.
Shobha is a liar.

Shobha
I am the singer.
Teena is not a singer.

Teena
Shobha is the singer.
Kavita tells a lie.

Let's take Kavita to be the truth-teller. 
That would make Kavita the singer and Shobha the liar. 
Shobha being the liar says Teena is not a singer, meaning Teena would actually be a singer. Therefore, this scenario fails. 
This make Teena an alternator, 
If Teena's first statement is false, that must mean that the second statement is true, which says that Kavita tells a lie, but our assumption was that Kavita is the truth teller. 
Therefore, this scenario fails. 

Let's take Kavita to be the Liar. 
That would give us that Kavita is not the singer and Shobha is not a liar. 
Now, let's take Shobha to be the truth-teller. 
This gives that, Shobha is the singer and Teena is not a singer. 
Now, Teena must be the alternator, The first statement is true, so that must make the second statement false, that Kavita tells a lie. 
Kavita is a liar and tells two lies. This satisfies our conditions. 

We can try the other scenario's as well. 
Taking Kavita to be the liar, and Teena to be the truth-teller. 
Making the first statement true and second false, but that breaks down as we know Kavita tells two lies and not one. 

Furthermore, once we found out that Shobha can be the singer, we need not check for other case as we do not have to find all the cases possible and Shobha is in a single option, and we can mark that as our answer. 
Therefore, Option D is the correct answer.