Fill in the blank with the most appropriate word.
It was a wonderful holiday. I will always remember _____ the Niagara Falls.
Option A is the correct answer as the verb 'remember' requires an object to complete its meaning and gerund 'seeing' is serving this purpose. Also, the sentence talks about the activity that happened in the past. So, a gerund, not an infinitive, can convey this sense best. This is why Option B cannot be used as it is an infinitive. Options C and D cannot be used as this sentence does not require a verb to complete its meaning.