The correct word is "thick". There is a popular simile which uses it: "as thick as pea soup". Although, thanks to the popularity of the animated Christmas show 'Rudolph the Red Nosed Reindeer', another simile is "as thick as peanut butter".

A thick fog. A very thick fog with pollution is called "a pea-souper", ie. it's as thick (and yellow) as pea soup.

i think thick

Thick.