Select the most appropriate word to fill in the blank.

Please don't disturb me. I ______ to solve a sum.

am trying

The most appropriate word to fill in the blank is am trying.

The sentence is in the present continuous tense, which is used to talk about something that is happening now. The speaker is asking the listener not to disturb them because they are currently trying to solve a sum.

The other options are not as appropriate.

• The word "try" is the simple present tense of the verb "try," and it is used to talk about something that happens regularly or that is happening now. However, it does not convey the sense of urgency that the speaker is trying to convey.
• The word "tried" is the past tense of the verb "try," and it is used to talk about something that happened in the past. It does not make sense to say that the speaker is trying to solve a sum that they have already tried to solve.
• The word "have tried" is the present perfect tense of the verb "try," and it is used to talk about something that has happened in the past but has a bearing on the present. It does not make sense to say that the speaker is trying to solve a sum that they have already tried to solve and failed.