Select the sentence that uses the given idiom correctly.
Skating on thin ice

Correct Answer: He was on thin ice after making a controversial statement.

Explanation:
The idiom "skating on thin ice" (or being "on thin ice") means to be in a risky, precarious, or dangerous situation where a mistake or further action could lead to serious trouble or negative consequences.

Let's evaluate how the phrase is used in each sentence:

1. "They were on thin ice when they skated on the frozen lake." — This sentence uses the phrase literally (actual ice on a lake) rather than idiomatically.
2. "She was on thin ice while trying to balance on a tightrope." — This incorrectly mixes the idiom with literal physical balancing on a tightrope.
3. "He was on thin ice after making a controversial statement." — Correct. Making a controversial statement creates a figuratively risky situation where he risks severe criticism or punishment, which accurately reflects the meaning of the idiom.
4. "They were on thin ice due to the icy conditions on the road." — This also uses the phrase literally regarding weather and road conditions.

Hence, the sentence that uses the idiom correctly is "He was on thin ice after making a controversial statement."