Identify the demonstrative pronoun in the following sentence? This is the friend i was telling you about

The answer is "this." A demonstrative pronoun takes the place of a noun phrase.

For example:

Those cookies you are baking smell delicious.

A demonstrative pronoun could take the place of the bold phrase. So, we could instead say something like:

They smell delicious.

or

Those smell delicious.

In the sample I gave you, they and these are demonstrative pronouns.