The correct option is A slips - slipped
The context refers to an action in past continuous tense. The form of the sentence is S + was + (verb + ing) + O. Thus, the second verb will also follow the same tense.
The error is in option A. "Slips" should be replaced with "slipped" as it indicates an action of the past to concur with the tense of the sentence.
Option B "trying" is the correct form of the verb (try + ing = trying).
Option C "climb" is correct. "Climbing" indicates the present continuous tense which means an action that is taking place at the time of speaking.
Option D "was" indicates the action in past as reported in a magazine, the context implies that this is a retelling of something that has already occurred.
Thus, option A show the error and is the correct choice.