Unfortunately, corruption is the … of most African countries.

Analyze the question: We need the word that best completes "corruption is the ___ of most African countries." The word "Unfortunately" tells us the sentence is negative, so we need a word that describes something harmful or a source of misery.

Explain the concept: This is a vocabulary-in-context question. Here are the meanings:

• Venom: poison, usually from a snake, or figuratively, spite. We don't say something is "the venom of a country."
• Bane: a cause of continual distress or ruin. It is often used in the phrase "the bane of someone's/something's existence."
• Disaster: a sudden calamity. It doesn't fit the idiom "the ___ of" here, and corruption is an ongoing problem, not a single event.
• Backbone: the main support or strength of something. It is positive, so it contradicts "Unfortunately."

Solution:

1. "Unfortunately" signals something negative, so we eliminate D (backbone), which is positive.
2. "The ___ of" is a possessive-type structure. "Venom" and "disaster" don't collocate naturally with it.
3. "The bane of" is a standard collocation meaning the thing that causes persistent trouble.

The sentence becomes: Unfortunately, corruption is the bane of most African countries.

Tip: Memorize the common collocation "the bane of." Also, in "best completes" questions, watch the signal words (Unfortunately, Fortunately, However) because they tell you whether the missing word is positive or negative.