A microscope that is calibrated this way will maintain the specimen in focus when the objective lens is rotated. ● Pointer
● C-mount
●fine focus
●Parfocal

The correct option is;

Parfocal

Explanation:

A perfocal lens is one that when in use, the object remains in focus when the  objective lens is rotated to change the magnification power of the microscope. That is the when the magnification is adjusted, the objectives remains in focus. The use of the parfocal lens however results in a small but insignificant amount of error in the focus.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

the backbone of the dna and rna compound is sugar phosphate.