Update and limit the number of nested attributes in Ruby on Rails

I have a Question model related to Option model. Whenever I try to update question and related options, new options are created instead of updating an existing option. Why is it happening like this? Also I would like to display only four options in the form, no less no more. How can I implement it in my form?

My code is as below:

Models

class Question < ApplicationRecord
    has_many :options, dependent: :delete_all, :autosave => true
    validates_length_of :options, maximum: 4
    accepts_nested_attributes_for :options
end
class Option < ApplicationRecord
    belongs_to :question
    validates_associated :question
end

_form.html.erb

<%= form_with(model: question, local: true) do |form| %>
    <div class="field">
        <%= form.label :body %>
        <%= form.text_area :body %>
    </div>

    <%= form.fields_for :options do |a| %>
        <div class="field">
            <%= a.label :options %>
            <%= a.text_area :body %>
        </div>
   <% end %>


   <div class="actions">
       <%= form.submit %>
  </div>
<% end %>

controller

class QuestionsController < ApplicationController
    before_action :set_question, only: [:show, :edit, :update, :destroy]
    before_action :authenticate_user!, except: [:show, :index]
    def edit
    end
    def update
        respond_to do |format|
            if @question.update(question_params)
                format.html { redirect_to @question, notice: 'Question was successfully updated.' }
                format.json { render :show, status: :ok, location: @question }
            else
                format.html { render :edit }
                format.json { render json: @question.errors, status: :unprocessable_entity }
            end
        end
    end

    private 
        def set_question  
            @question = Question.find(params[:id])
        end

        def question_params
            params.require(:question).permit(:body, options_attributes: [:body])
        end
end

I have used scaffold for questions. Thanks in advance!

Solution?