Please check this excercise and correct if wrong.

My sister, who lives in Los Angeles, is a designer.
a) I have one sister.
b) I have more than one sister.

a) is correct because non-restrictive clasue.

The athletes who tested positive for doping were suspended.
a) All the athletes tested positive for doping.
b) Some athletes didn’t test positive for doping.

a) is correct.

Flat screen televisions, which are very expensive, have a better image quality.
a) All flat screen televisions are very expensive.
b) Some flat screen televisions are cheap.

a) is correct.

The teachers at this school who arrive late for class will be suspended.
a) Only some teachers arrive late for class.
b) All the teachers in the school arrive late for class.

a) is correct.

The company president who works in Berlin is moving to London.
a) There is only one company president.
b) There is more than one company president.

b) is correct.

I would say one of your answers is incorrect. You clearly understand the rule so take another look.
Teachers: We supply a list of EFL job vacancies
Dear teacher!

I think question (2) is incorrect.

You got it!