Sithu
becaus he
he
(work out) at the gym tomorrow
is training for a
marathon,

Sithu will be working out at the gym tomorrow because he is training for a marathon.

Explanation:

Here, we need to choose the correct tense. There are two tenses we could potentially use:

Present continuous - usually used to talk about events that are taking place right now. However, we can use it to talk about things that have already been decided or plans that have already been made for the future. For example, Mary is meeting her best friend tonight. Mary knows when and where exactly she is meeting with her friend. This is an arrangement.Future continuous - used to talk about events that will happen at a particular moment in the future and continue for an expected length of time. We use it to talk about decisions we've made about what we will do in the future, but not precise arrangements.

As Sithu probably didn't make a plan about when exactly he's going to be working out, it would be best to use the future continuous tense - will be working out. He made a decision, not an arrangement.