Members of the algal class Phaeophyceae are commonly known by which of the following names?

Introduction / Context:
Algae are divided into different groups based on pigments, stored food, and cell wall composition. Phaeophyceae is one of the main classes within the algae and is commonly discussed in botany and general biology. This question asks for the popular name used for members of Phaeophyceae, which helps you link scientific classification with everyday terminology.

Given Data / Assumptions:

• The algal group of interest is Phaeophyceae.
• Options list green, brown, red and yellow algae as possible common names.
• We assume standard classification based on pigment composition.
• Examples of Phaeophyceae include seaweeds such as kelps and rockweeds.

Concept / Approach:
Phaeophyceae mainly contain brown pigments such as fucoxanthin in addition to chlorophyll, which give them a characteristic brown or olive green colour. They are predominantly marine and often form large seaweeds. Because of their pigment composition and colour, they are commonly called brown algae. In contrast, Chlorophyceae are called green algae, Rhodophyceae are called red algae, and there is no widely used major group known simply as yellow algae in basic textbooks.

Step-by-Step Solution:
Step 1: Recall that algae are often informally grouped as green, brown and red based on their dominant pigments.Step 2: Associate Phaeophyceae with fucoxanthin pigment, which imparts a brownish colour, leading to the name brown algae.Step 3: Recognize that Chlorophyceae corresponds to green algae and Rhodophyceae to red algae, not to Phaeophyceae.Step 4: Evaluate option B, brown algae, which directly matches Phaeophyceae.Step 5: Reject options A and C because they refer to other algal classes, not Phaeophyceae.Step 6: Reject option D, yellow algae, because it is not the standard common name used in most basic texts for Phaeophyceae.Step 7: Conclude that Phaeophyceae are commonly called brown algae.

Verification / Alternative check:
In botany textbooks, examples such as Laminaria, Sargassum and Fucus are listed under Phaeophyceae and are described as large brown seaweeds. These same books categorize green freshwater forms like Chlamydomonas and Spirogyra under green algae and red seaweeds like Polysiphonia under red algae. This consistent usage confirms that the common name for Phaeophyceae is brown algae.

Why Other Options Are Wrong:
Option A is wrong because green algae refers to Chlorophyceae, not Phaeophyceae. Option C is wrong because red algae refers to Rhodophyceae. Option D is wrong because yellow algae is not the standard common name associated with Phaeophyceae in typical classification schemes.

Common Pitfalls:
Students sometimes confuse the Latin class names because they are unfamiliar, especially when all end with similar suffixes. A useful strategy is to associate Phaeophyceae with the word phaeo, meaning dusky or brown, and Rhodophyceae with rhodo, meaning red. Linking the root of the word to the colour helps you quickly match each class with its common name. This approach makes it easier to answer questions that test your recall of algal classification.

Final Answer:
Brown algae.