What is the reason for the red color of red algae?

Ulothrix filaments produce . . . . . . .

Which of the following algae are used as food supplements by space travellers?

In which of the following plants is the sporophytic generation represented only by the zygote?

Assertion : Red algae contribute to the production of coral reefs.
Reason : Some red algae secrete and deposit calcium carbonate over their walls.

Match the following classes of algae with their characteristic colours:
$A.$ Cyanophyceae $a.$ Green colour
$B.$ Chlorophyceae $b.$ Blue-green colour
$C.$ Phaeophyceae $c.$ Red colour
$D.$ Rhodophyceae $d.$ Brown colour