No sé si voy a ir a la playa.
a. es mejor que vayas a la playa.
b. es mejor que vas a la playa.
c. es mejor que vayamos a la playa.

Let's see this as a conversation between two persons, where the first person says:

No sé si voy a ir a la playa.  

I do not know if I'm going to the beach.

To express its doubt in making the decision to go or not go to the beach (uncertainty).

Then the second person gives its opinion, and this is where the subjunctive mood of a verb comes in:

The subjunctive mood is usually used in sentences to express desires, orders, emotion, possibility, judgment, opinion, necessity, or statements that are contrary to facts in the actualities, it is also called the mode of unreality.

That is, the subjunctive expresses uncertainties, subjectivities and possibilities and is presented as a possible, probable or hypothetical event.

What does this mean?  

This means that the opinion of the second person respect to the doubt of the first person must be expressed in subjunctive mood.

So, according to this explanation, the correct option is:

It is better that you go to the beach

This is because here the verb ir (to go) is written in present and subjunctive mood as vayas.

The other options are incorrect because:

-Es mejor que (tú) vas a la playa, although it is in present tense, it is in indicative mood.

-Es mejor que (nosotros) vayamos a la playa, although it is in present tense and subjunctive mood, the verb is not conjugate according to the subject tú (you). In this case is conjugated with the subject nosotros (we).