Choose the correct indefinite pronoun from the options given below:
Indefinite pronouns are used for non-specific things (anybody, somebody etc).
The pronoun used in the sentence is 'I' as part of 'I'll" which is 'I will'.
The subject in the sentence says " I'll have it" referring to the piece of cake.
From the options given, if 'someone' has the cake, the pronoun used cannot be 'I' because somebody is used to indicate a third non-specific person. Therefore, option A) someone is incorrect.
If 'anyone' wants the piece of cake, the subject cannot eat the cake either as 'anyone' is also used in case of a non-specific person. Therefore, option B) anyone is incorrect.
The same holds true for 'anybody' i.e option D) which is also the incorrect option.
If 'no one' wants the piece of cake, then the subject can have it. Therefore, option C) no one is the correct answer that completes the sentence meaningfully.