Mosquito parasites have become … to quinine.

Step 1: Analyze the question The sentence is: "Mosquito parasites have become ...... to quinine." We need the word that correctly fits both meaning and the preposition "to".

Step 2: Explain the concept This tests word choice and collocation (words that naturally go together). Let's look at each option:

• Resistant to: means able to withstand the effect of something. We say "bacteria are resistant to antibiotics" and "parasites are resistant to drugs." This is the standard phrase for organisms and drugs.
• Immune to: this can mean protected from a disease or effect. It is usually used for a body's protection against disease ("immune to measles"). Parasites are not typically described as "immune" to a drug; the scientific term for drug-surviving organisms is "resistant."
• Opposed to: means against an idea or plan ("opposed to the proposal"). It does not fit a biological situation.
• Defiant: means openly disobedient. It is used with people, and it takes "of" or no preposition, not "to," in this sense.

Step 3: Solution Quinine is an antimalarial drug. Over time, malaria parasites carried by mosquitoes have developed the ability to survive its effects. In science and medicine, this is called drug resistance. So the correct sentence is:

"Mosquito parasites have become resistant to quinine."

A note on "immune": some might be tempted by it, but it is a weaker fit here. "Immune" describes a host's defence system, while "resistant" describes organisms that withstand a drug.

Answer: A. resistant

Exam tip: Remember the pairing "resistant to" + drug/antibiotic/chemical and "immune to/from" + disease. When a question is about germs or parasites surviving a medicine, think "resistant."