What is an effector?
a. a molecule that brings about a cellular response to a signal.
b. a molecule that brings a signal to a cellular response.
c. a molecule that brings enzymatic changes to a cell.
d. none of these is the correct answer.

The correct answer is a A molecule that brings about a cellular response to a signal.

Explanation:

Effector molecule is a small molecule that act as ligand to generate various cellular response by binding with target proteins(receptors).

  Effector molecules can regulate catalytic activity of enzymes,gene expression and various signaling processes that are very much important for proper functioning of the cell that the effect molecule target.

 Examples Allosteric effectors which can be either modulators or inhibitors depending on the nature of the effector molecule.

there is none because earth is stationary and flat.