Sentence improvement: She glanced back to make sure she ______ observed. Which option correctly and naturally completes the bracketed part?

Introduction / Context:
This sentence improvement question checks your control over passive continuous verb forms and subject verb agreement. The sentence describes a person looking back to check whether anybody is watching her at that moment. English uses "was not being observed" to show an action in progress in the past in the passive voice.

Given Data / Assumptions:
- Base sentence: "She glanced back to make sure she (wasn't to be) observed." - Subject of the clause is "she", a singular third person. - The intended meaning is that she wanted to be sure no one was watching her at that time. - Four options show different verb forms and structures.

Concept / Approach:
In passive continuous forms for the past, we use "was" or "were" plus "being" plus past participle. Here, the verb "observe" becomes "observed" in passive. Because the subject "she" is singular, the correct auxiliary is "was". The negative form is "was not being observed". The phrase "wasn't to be" suggests obligation or destiny and does not fit the context of checking for current surveillance.

Step-by-Step Solution:
Step 1: Identify that the situation is in the past and in progress: she glanced back while a possible act of observing might be happening. Step 2: Choose passive continuous: was being observed. Step 3: Add negation: was not being observed. Step 4: Match with the option that gives "wasn't being". Option C corresponds exactly to this structure. Step 5: Reject "wasn't be" because it is not a valid verb form. Step 6: Reject "weren't being" because "were" does not agree with singular "she". Step 7: Reject "no improvement" because "wasn't to be observed" expresses a different idea about intention or fate, not about ongoing action.

Verification / Alternative check:
Consider a similar sentence: "He looked around to make sure he was not being followed." This is the standard pattern used in stories and conversations. Replacing "followed" with "observed" gives "was not being observed". The suggested answer fits this pattern exactly. The original construction "was not to be observed" would mean that she was not supposed to be seen by anyone, which is a different nuance and sounds unnatural in this specific sentence.

Why Other Options Are Wrong:
Option A, "wasn't be", breaks English grammar because "was" cannot be directly followed by a bare infinitive "be" in this way. Option B, "weren't being", uses the plural auxiliary "were" with singular "she", causing disagreement. Option D, "no improvement", keeps the unnatural modal like structure "wasn't to be observed", which does not express the intended ongoing surveillance meaning.

Common Pitfalls:
Students sometimes confuse the patterns "be to" (as in "He is to leave tomorrow") with continuous passive forms. Remember that "was to be observed" indicates something like a plan or obligation, while "was being observed" indicates an action in progress. When you see clues like "to make sure" and a story like description, continuous passive is often the best fit.

Final Answer:
The correct improvement is "She glanced back to make sure she was not being observed."