Among the following, which bird is considered the smallest in size?

Introduction / Context:
This question belongs to basic general science and zoology. It asks about the smallest bird among common examples. Birds differ greatly in size, from very large flightless birds to tiny species that can hover in the air. Knowledge of extremes in nature, such as the largest or smallest animals, is a frequent topic in competitive examinations because it provides easy to test factual recall and helps build a sense of biological diversity.

Given Data / Assumptions:
• Four birds are listed: Ostrich, Hummingbird, Kingfisher, and Sparrow. • The task is to identify the smallest bird in terms of body size. • All four are real, well known birds, making the question a straightforward comparison. • No additional context such as habitat or region is required. • Only one option is correct for being the smallest among those listed.

Concept / Approach:
Among birds, hummingbirds are widely recognized as some of the smallest, with certain species measuring only a few centimeters in length and weighing just a few grams. In contrast, ostriches are the largest living birds, kingfishers are small to medium sized birds, and sparrows are small but still significantly larger than the tiniest hummingbirds. The approach is therefore to recall that hummingbirds hold records for minimal size and weight, which immediately distinguishes them from the other options.

Step-by-Step Solution:
1. Consider the approximate size of an ostrich, which can stand more than two meters tall and is obviously very large. 2. Recall that kingfishers, although smaller than many birds, have sturdy bodies and are not extremely tiny when compared to the smallest birds known. 3. Think about common house sparrows, which are small, commonly seen birds but still several times larger than the very smallest bird species. 4. Remember that hummingbirds, particularly species like the bee hummingbird, are often described as the smallest birds on Earth. 5. Among the options provided, hummingbirds clearly fit the category of tiniest birds by both length and mass. 6. Therefore, select option b, Hummingbird, as the correct answer.

Verification / Alternative check:
Verification can be done by recalling general facts about animal records: most school textbooks and children's encyclopedias mention the ostrich as the largest bird and the hummingbird, especially the bee hummingbird, as the smallest. Even without exact measurements, it is clear that kingfishers and sparrows do not hold any record for minimal size. Comparing images or exhibits in zoos and natural history museums would also immediately show that hummingbirds are much smaller than sparrows or kingfishers.

Why Other Options Are Wrong:
• Ostrich is actually the largest living bird, not the smallest. • Kingfisher is a small to medium sized bird known for catching fish, but it is much larger than a hummingbird. • Sparrow is a small bird commonly seen in towns and villages, yet its size is still significantly greater than that of the smallest hummingbirds.

Common Pitfalls:
Some candidates may choose sparrow because it is a very familiar small bird and they may not have personally seen a hummingbird. Others might be confused by the names and assume that kingfishers could be very small because they perch on wires. Another mistake is not remembering that hummingbirds are native to the Americas and therefore may be less familiar to people in other regions. To avoid such errors, it is helpful to remember basic animal records: ostrich is the largest bird, and hummingbird, specifically the smallest species among them, holds the record for smallest bird.

Final Answer:
Among the birds listed, the smallest in size is the Hummingbird.