Which is the subject of this sentence?
for once in your life, try something new and exciting.
a. an understood you
b. your
c. something
d. life

For once in your life, try something new and exciting.

The subject of the above sewntence is A. an understood you.

The sentence talks about you and the speaker is talking to you.

The above sentence is a compound sentence. It has an independent clause and a dependent clause.

The independent clause is (You) try something new and exciting.
The dependent clause is For once in you life. It starts with a coordination conjunction and it cannot stand on its own.