。
The yakuza are putting Hiroshi through a trial.
Literal
Yakuza [subject-が] Hiroshi [target-に] trial [object-を] imposing [progressive-ている].
課す ('to impose, levy, assign') is a formal verb used for imposing tasks, duties, trials, or taxes. 課している is the progressive form. やくざ is written in hiragana (normally ヤクザ in katakana or 893 as numerical pun). The に marks Hiroshi as the target/recipient of the imposition.