You can use either one but it will change the meaning of the sentence. If you said "pleased" you're saying that Mary was happy to either be next to you or was happy to be sitting down. If you said "pleasant" you're saying that you enjoyed Mary's presence, that she was a nice person. Pleased is the past-tense conjugation of the verb "to please" or "to make happy" Pleasant is an adjective similar to nice. Example: "It was a pleasant night."

http://www.usingenglish.com/forum/ask-teacher/52714-pleasing-pleasant-pleased.html As adjectives: Pleasing = the act of giving pleasure/agreeable: That was a pleasing film we saw last night. She is a pleasing person to spend an evening with. Pleasant = Having qualities that give pleasure: It was a pleasant evening. The meal was very pleasant. Pleased = Feeling or showing satisfaction. I was so pleased to meet him. Are you pleased that your brother is engaged to be married?

You would have to say pleased. pleased means that you ARE HAPPY WITH something, but pleasant means that something IS nice. So if you said pleasant, you would just be saying that Mary is nice, which isn't really the point in this sentence. good luck :)

说跌死捏。。。。。May I ask why a lot of people attention you pay no attention to me