Vocabulary, Word Choice and Register
What you must know
Word choice is judged on precision and fitness for audience. Understand denotation, the dictionary meaning, and connotation, the associations a word carries: 'thrifty' and 'stingy' describe similar behaviour but judge it differently. Learn to select synonyms that fit context, use collocations naturally ('make a decision', not 'do a decision'), and match register: formal writing avoids slang, contractions and vague intensifiers such as 'really' or 'a lot'. Recognise persuasive colouring in adjectives, euphemism ('passed away'), jargon, and emotive language.
Traps
The classic trap is choosing a synonym with the wrong shade of meaning or level of formality, such as 'kids' in a formal report or 'commence' in a friendly email. Beware malapropisms, redundancy ('return back'), and overwriting: three ornate adjectives rarely beat one exact one. In 'replace the underlined word' questions, re-read the whole sentence after substitution to check it still sounds natural.
Worked example
Sentence: 'The council's plan was described as childish.' If the intended meaning is innocent and playful, 'childish' is wrong; 'childlike' carries no criticism. The denotation is close, but the connotation decides the mark.
Rule of thumb
Say your replacement sentence aloud as if to the intended reader. Would they hear the tone you meant?