Tesla Inc. TSLA, +1.95% Chief Executive Elon Musk exercised options on 175,000 shares in the automobile company, adding to his stake at prices less than one-third the going rate. Musk's options on more than 5 million Tesla shares matured years ago, and he previously exercised more than 500,000 options at a lower price. According to a filing with the Securities and Exchange Commission, Musk exercised the fresh shares Monday at a price of $31.17, paying the cost of exercising the options -- nearly $5.5 million -- as well as the taxes without selling other shares. Musk still has access to about 4.6 million vested options, according to the SEC filing, and they expire in a little more than three years. Before Monday's option exercise, Musk owned 19.5% of Tesla stock, according to FactSet. Tesla stock closed Tuesday at $205.08, after declining about 29% in the past 12 months.