Error Spotting – Choose the part (A–D) that contains a grammatical error, or choose No error if the sentence is fully correct. Sentence (parts correspond to options below): A) Reasonable ambition, if supported B) at persistent efforts, C) is likely to yield D) the desired results.

Introduction / Context:
This question checks preposition choice with support. The idiomatic preposition after supported in this sense is by, not at, when indicating the means or agent that provides support.

Given Data / Assumptions:

• Target collocation: supported by
• Current phrase: supported at persistent efforts
• Intended meaning: efforts provide the support

Concept / Approach:
In passive-like constructions, by commonly introduces the agent or the means. Therefore, supported by persistent efforts is the correct phrasing. Using at mis-specifies the relationship and sounds non-idiomatic.

Step-by-Step Solution:

Verification / Alternative check:
Parallel examples: sustained by hard work; powered by solar energy. The by-phrase introduces the source or agent of support.

Why Other Options Are Wrong:

• A) Conditional modifier is grammatical.
• C) Predicate is acceptable and agrees with the singular subject ambition.
• D) Object noun phrase the desired results is idiomatic.

Common Pitfalls:
Prepositions are often confused due to first-language influence. Memorize core collocations such as dependent on, interested in, and supported by.

Final Answer:
Option B contains the error. Use by instead of at.