Which of the following is a unicellular and small alga?

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

Which of the following organisms cannot fix nitrogen?
$A.$ Azotobacter
$B.$ Oscillatoria
$C.$ Anabaena
$D.$ Volvox
$E.$ Nostoc
Choose the correct answer from the options given below:

Hormogonia are the vegetative reproductive structures found in:

Cells of $Spirogyra$ are

What is the term used for a plant body that lacks tissues and organs?