According to newton’s first law of motion, it is true that an object moving at a constant speed in a straight path will continue to do so until a net force acts upon it.
a. true
b. false

In Newton's First Law of Motion, it is stated : 
"an object in motion will stay in motion and in the same direction until acted upon by an opposing force."
It is also applied to objects NOT in motion :
"An object at rest will stay at rest until acted upon by an unbalanced force."
Your question asks if "According to newton's first law of motion, it is true that an object moving at a constant speed in a straight path will continue to do so until a net force acts upon it. True or false?"
This is true.
I hope this helps!