Without any calculations it's evident it can't be neither B (both numbers are even, so they're divisible by 2) nor C (the numbers end in 0 and 5, so they're divisible by 5).
A.

Both numbers have a factor of 3, so they're not relatively prime.
That means it must be D. But, let's check it.

Indeed, those two numbers are relatively prime.
It’s 1
...................
Answer: The final number is 5
Step-by-step explanation:
Picked number: 10
STEP are all according to what you stated
10+5=15 (first sentence)
15+10=25 (second sentence)
25+5=30 (third sentence)
30÷2-10=5 (fourth sentence)
The final number is 5.