The correct answer is A. Discover.
To discover is to find something that was previously unknown. It can also mean to learn something new. For example, Christopher Columbus discovered America in 1492.
To invent is to create something new. It can also mean to come up with a new idea. For example, Thomas Edison invented the light bulb in 1879.
To determine is to find out something by using evidence or reasoning. It can also mean to decide something. For example, a judge might determine the guilt or innocence of a defendant in a court case.
To fix is to repair something that is broken or damaged. It can also mean to make something work properly. For example, a mechanic might fix a car that is not running properly.
In the context of the question, the word “detect” means to find something that was previously unknown. This is most closely related to the word “discover.”