(A) Immediately stop investing in marketing for the source.
(B) Immediately increase marketing spend for the source.
(C) Determine whether the traffic source fulfills other site goals.
(D) Determine whether the source assists in conversions.
If one of your paid traffic sources has a low conversion rate then you should not ignore and stop investing more money in marketing for that source. Instead, you should first check whether the traffic source fulfills other goals like sending targeted leads to your product pages, have high click-through rates, etc.
The next thing is to check if the traffic source is assisting in conversion by analyzing the Assisted Conversions report. You can find if the traffic source appeared on a conversion path. If the traffic source appears in the conversion path or fulfills other goals then you should keep on investing in the source otherwise it is better to leave that source.
This question is a part of the Assessment 2 section of the Google Analytics for Power Users course. You can find all the questions from all the four assessments with detailed explanation for this exam in our Google Analytics for Power Users Assessment Answers page.