Questions modelo you hear: les vas a servir sopa a los clientes? you read: sí, se la voy a servir. no, no se lo voy a servir. you select: sí, se la voy a servir.

For this case we have the following expression:

¿Les vas a servir sopa a los clientes?

In the question we observe that "los clientes" is referred to the third person of the plural.

However, as we speak of "la sopa" we mean the third person singular.

Therefore, the answer must be given to the third person singular.

We have then:

Sí, se la voy a servir.

Sí, se la voy a servir.