Final answer:
The correct -ísimo form of the adjective aburrido is aburridísimo.
Step-by-step explanation:
The correct -ísimo form of the adjective aburrido would be aburridísimo.
When an adjective ends in a vowel, we typically add -ísimo to make the superlative form. In this case, since aburrido ends in -o, we remove the -o and add -ísimo.
For example, otro (another) becomes otroísimo (the most another) and complicado (complicated) becomes complicadísimo (the most complicated).