A series of types of angles is given with one term missing. Select the correct alternative from the options that will logically complete the series. Acute angle, Right angle, Obtuse angle, ?

Introduction / Context:
Questions that list different types of angles test a candidate's understanding of basic geometry vocabulary and the classification of angles by their measure. In this series, each term represents a well known category of angle. To find the missing term, we must identify the pattern that links the given types and then choose the angle that logically follows in that pattern. This is a standard conceptual reasoning task based on angle measures in degrees.

Given Data / Assumptions:

• Given series: Acute angle, Right angle, Obtuse angle, ?
• Known angle ranges:
  • Acute angle: greater than 0 degrees and less than 90 degrees.
  • Right angle: exactly 90 degrees.
  • Obtuse angle: greater than 90 degrees and less than 180 degrees.
  • Straight angle: exactly 180 degrees.
• We look for a simple, increasing sequence in terms of angle measure.

Concept / Approach:
The concept here is classification of angles on the basis of their size. If we observe the given three terms in order, they move from smaller to larger: an acute angle is the smallest among the listed categories, a right angle is a specific middle value at 90 degrees, and an obtuse angle is larger than 90 degrees but still less than 180 degrees. The natural next step in this increasing sequence of angle sizes is the angle that measures exactly 180 degrees, which is called a straight angle.

Step-by-Step Solution:
Step 1: Identify the measure range for an acute angle: it is more than 0 degrees and less than 90 degrees. Step 2: Identify the measure for a right angle: it is exactly 90 degrees and is therefore larger than any acute angle. Step 3: Identify the measure range for an obtuse angle: it is greater than 90 degrees but less than 180 degrees, so it is larger than a right angle. Step 4: Look for the next natural category after angles less than 180 degrees. The next standard type is the straight angle, which has a measure of exactly 180 degrees, so Straight angle completes the increasing sequence of angle measures.

Verification / Alternative check:
One way to verify the logic is to list the common types of angles in increasing order of measure: acute, right, obtuse, straight, reflex, and then a complete revolution. The given series matches the first three entries of this list exactly. Therefore, the missing fourth term should be the next standard type, namely the straight angle. This confirms that Straight angle is consistent both with the known classification and the pattern of increasing angle size.

Why Other Options Are Wrong:
Exterior angle is a relative term used in polygon geometry and does not fit into this simple measure based sequence. Reflex angle is any angle greater than 180 degrees and less than 360 degrees, which jumps beyond the immediate next category after obtuse. Supplementary angle is not a single angle type; it refers to a pair of angles whose sum is 180 degrees, so it does not represent a single position in the series. Complementary angle also refers to a pair of angles that sum to 90 degrees, not a single angle type in the classification chain. Hence, these options do not satisfy the required pattern.

Common Pitfalls:
Students sometimes confuse supplementary and straight angles because both are associated with 180 degrees. However, a supplementary pair contains two angles that add up to 180 degrees, while a straight angle is a single angle that measures 180 degrees. Another mistake is to choose reflex angle just because it is a well known term, without checking whether it is the next angle type after obtuse in the standard ascending order. Always think in terms of angle measurement when dealing with such series.

Final Answer:
The series lists angles in increasing order of their measure. After acute, right, and obtuse angles, the next standard type is the Straight angle with a measure of 180 degrees.