Which word best completes the sentence: If you are ______ with this test, perhaps you should attempt an easier one. a) stumbling b) stressed c) struggling d) straining

The correct option is c.

The sentence implies that if you find the test difficult to complete or understand, you should try an easier one.

• a) stumbling: To stumble means to trip or make a mistake. While you might stumble on a question, "stumbling with this test" isn't the most natural fit for overall difficulty.
• b) stressed: To be stressed means to feel mental or emotional pressure. While a difficult test can cause stress, the sentence focuses on the act of performing the test, not just the emotional state. "Stressed with this test" is also grammatically less common than "stressed by/about this test."
• c) struggling: To struggle means to have difficulty doing something. "Struggling with this test" perfectly conveys the idea of finding the test challenging and having trouble completing it.
• d) straining: To strain means to make a great effort or to injure oneself by overexertion. While you might strain to understand a difficult concept, "straining with this test" is less common and less appropriate than "struggling" in this context.

Therefore, "struggling" is the best fit.

The completed sentence is: If you are struggling with this test, perhaps you should attempt an easier one.

The final answer is $\boxed{\text{c. struggling}}$.