In the following passage, choose the most appropriate adverb to complete the sentence: “The purple and bronzed grackles (the latter are ______ seen on the plains of Colorado, but are not common) are mentioned in contrast with Brewer blackbird.”

Introduction / Context:
This question tests the correct use of adverbs of frequency, particularly “sometime” versus “sometimes.” The passage describes how often certain birds, namely the bronzed grackles, are seen on the plains of Colorado. To express an occasional, not very frequent occurrence, English uses specific adverbs, and we must choose the one that fits both grammar and meaning.

Given Data / Assumptions:
The sentence fragment is: “the latter are ______ seen on the plains of Colorado, but are not common.”
We are choosing between “sometime,” “sometimes,” “at time,” and “any time.”
The clause “but are not common” indicates that the birds appear only occasionally.
We assume that the sentence is written in standard English and follows normal rules for adverbs of frequency.

Concept / Approach:
“Sometimes” means “on some occasions” or “now and then” and is used to talk about actions that happen occasionally. “Sometime,” however, usually refers to an unspecified point in time in the future or past, as in “We should meet sometime.” The other phrases “at time” and “any time” do not fit grammatically in the given structure. Therefore, we look for the adverb that describes occasional frequency and fits grammatically before the past participle “seen.” That adverb is “sometimes.”

Step-by-Step Solution:
Step 1: Identify the function of the missing word. It modifies the verb “are seen” and tells us how often the birds are seen. Step 2: Recall meanings: “sometimes” means “occasionally,” while “sometime” is an indefinite point in time. Step 3: Insert “sometimes”: “the latter are sometimes seen on the plains of Colorado, but are not common.” Step 4: Read the entire sentence. The combination of “sometimes” and “not common” gives a clear picture of rare but occasional sightings. Step 5: Test the other options and see that they either sound ungrammatical or do not convey the intended sense of occasional frequency.

Verification / Alternative check:
We can compare with other familiar sentences. For example, “Tigers are sometimes seen near the village” clearly means they are seen on some occasions, but not regularly. If we replaced “sometimes” with “sometime,” the sentence would sound odd and unclear. Similarly, “at time” is incorrect; the standard phrase is “at times,” which is not provided. “Any time” usually appears in expressions like “You can visit any time,” which has a different meaning and structure. These comparisons confirm that “sometimes” is the only correct adverb in this context.

Why Other Options Are Wrong:
“sometime” refers to an unspecified point in time, not repeated occasional events; it does not fit with “are seen” in this sense.
“at time” is grammatically incorrect; the usual idiom is “at times,” which is not given as an option.
“any time” does not work before “seen” in this sentence and would not convey the idea of rare occurrences on some occasions.

Common Pitfalls:
A common mistake is thinking that “sometime” and “sometimes” are interchangeable. They are not. Learners also sometimes overlook the surrounding words “but are not common,” which clearly hint at occasional sightings. Always read the entire sentence and ask what kind of information the missing word must supply: frequency, time, cause, or condition. Here, frequency is clearly required, making “sometimes” the correct choice.

Final Answer:
The correct adverb is sometimes, giving the sentence: “the latter are sometimes seen on the plains of Colorado, but are not common.”