Option 1: Indrawati - This option states that the river Indrawati is not a tributary of the Ganga. To determine the correctness of this option, we need to know whether the Indrawati river actually flows into the Ganga or not.

Option 2: Son - This option does not relate to the question as it asks about a different river altogether.

Option 3: Gomati - This option states that the river Gomati is a tributary of the Ganga. This information does not answer the question as the question asks for a river that is not a tributary of the Ganga.

Option 4: Yamuna - This option also does not answer the question as it states that the river Yamuna is a tributary of the Ganga.

Based on the given options, option 1 - Indrawati, is the correct answer as it says that the Indrawati river is not a tributary of the Ganga.