I believe that the answer is D) Tell others the negative things you think about yourself.
The question states 'what might prevent you from improving your self-concept'. Basically, they're asking 'what might hold you back from improving your self-concept'. Starting with the first option, focusing on your strengths and your positives will NOT hold you back, as it will improve your self-concept. The second option, forming relationships with people with high self-esteem can also improve your self-concept. This is because putting yourself around others that have a high self-esteem could motivate you. The third option, ignoring what other people think of you, is also a great way to improve your self-concept. Now, the last option, telling others the negative things you think about yourself could decrease your self-concept. In this case, I believe that the answer is the last option.