Translation:Which additional answers are possible?
If you can't use the Umlaut for any reason, you can write the ä as ae (ö as oe; u as ue). It was once written this way. Than it was written as an a with a little e above. This e devolved into the two dots.
"Welcher" is a determiner here. After determiner the adjecitve follows the weak or mixed declension. After "welcher" it follows the weak declension and in nom. sing. must be "zusätzlichen"
Declension chart: http://en.wiktionary.org/wiki/zus%C3%A4tzlich#German
Note that this only explains "welcher" as a determiner. It can also be a relative pronoun:
Determiner: Welches Auto wirst du kaufen?| Which car will you buy?
relative pronoun: Das Auto, welches ich kaufe, ist blau.| The car, which i'm buying, is blue.
"möglich" is "possible".
"likely" is not only possible, but has a relative high probability. That would be "wahrscheinlich" in German.
For example, if I saw that "Rain is possible tomorrow", there might be a 10% probability, while "Rain is likely tomorrow" would have a higher probability, maybe 60%.