The outermost tissue layer of a cnidarian is the
a. ectoderm.
c. endoderm.
b. mesoglea.

Ectoderm

In most Cnidarian, the outermost tissue layer or the layer which is located at the uppermost part is called the ectoderm.

Ecto meaning outside which is why it is called the ectoderm. 

And contracy, the endoderm is the innermost part of the cnidarian stratum while mesoderm is the mid-portion of the stated layers.

The first statement is