This is a Spanish expression that you need to memorize. This expression is always like this: <em>hace un mes que</em>. In English, this literally is translated as: <em>It's been a month that</em>. So in this way we can write some sentences as follows: 1. <em>H</em><span><em>ace un mes que</em> no te veo
<em>I have not seen you for one month</em>
2. <em>H</em></span><em>ace un mes que</em> llegaste a mi vida
The correct way to indicate "for one month" is D. hace un mes que. There isn't a lot that can be said about this because there is no explanation, really - it is an expression that has to be learned by heart, and it cannot be changed. The expression will always be <em>hace un mes que </em>(literally translated as - it's been a month that...).