No sé si debo ir a la playa o al cine. a. es mejor que vayas a la playa. b. es mejor que vas a la playa. c. vé a la playa.

a. Es mejor que vayas a la playa.

Explanation: The sentence is:

No sé si debo ir a la playa o al cine.

Which means something like:

I don't know if I must go to the beach or at the movie theater.

The options given are:

a)  Es mejor que vayas a la playa.

b)  Es mejor que vas a la playa.

c) Vé a la playa.

Where options a and b are almost equal, but "b" is not written correctly.

The difference between option a and c, is that a is a suggestion, and c is almost an order.

In this situation, the right option is the polite one, so the right option will be a.