I think the answer is infinitive.
Answer:
A blog explaining how to build a skateboard ramp
.
Explanation:
The three other options reveal a motive other than to inform.
The first option is intended to recruit volunteers.
The third option is intended to further agenda (the construction of a new community theater)
The fourth option is intended to entertain.
Only the second option, a blog explaining how to build a skateboard ramp, is motivated by a desire to inform readers.
