The correct option is A Each
The Distributive Adjective 'each' refers to each individual of a group. In the given sentence, the angles of the pentagon are referred to individually hence option A is correct. 'Every' is similar in meaning except that it refers to the group as a collection of the members/segments and does not refer to each member/segment individually, hence option C is incorrect since we are referring to each angle of the pentagon individually and not as a whole. Option B is used when there is a choice to be made between two things or two people. Since there isn't a choice being made in the given statement, option B is incorrect. Similarly, Option D is the negative form of 'either', meaning 'not either' when there are two choices to choose from, hence the option is incorrect.