Here are the correct answers for each of the questions:
Emma is on holiday, so I ... after her cat until she returns
b. am lookingDavid... of dressing up as a pirate for the Halloween party next Sunday
a. thinksCan you have a look at Ann's computer? She ... to fix it all morning and it still doesn't work
d. has been tryingIt ... have been John who answered the phone last night; I know there was no one else at home
b. mustSusan ... abroad before and she was very excited before her first trip to Europe
c. had never beenI will never forget ... Saint Petersburg for the first time
c. visiting
Good luck with your studying!