from deduction rules http://aptitude.gateoverflow.in/2346/cat1999-125#a2370
$\text{Option (A)}:$
- Dinosaurs are prehistoric creatures , dinosaurs distributed term , prehisoric creatures non distributed $(2.1)$
- Water-buffaloes are not dinosaurs.here water buffaloes non distributed , dinosaurs distributed $(2.4)$
Given conclusion : Water-buffaloes are not prehistoric creatures. $\textbf{which is not correct,}$ here prehistoric creature should be non distributed bcoz in premise it is non distributed but in conclustion from $2.4$ prehistoric creature is distributed so $\text{it violate 7th rule}$
$\text{Option (B)}:$
- All politicians are frank.
- No frank people are crocodiles.
Given conclusion : "No crocodiles are politicians" $\textbf{this is correct,}$ following all rules$\dots$
$\text{Option (C)}:$
- No diamond is quartz.
- No opal is quartz.
Given conclusion : Diamonds are opals. here no conclusion exist beoz of $\text{rule (5)}$ if both premises are negative then no conclusion. so, $\textbf{it is not correct.}$
$\text{Option (D)}:$
- All monkeys like bananas , monkey distributed , bananas not distributed $(2.1)$
- Some GI Joes like bananas GI ,bananas both are not distributed $(2.2)$
Given conclusion : Some GI Joes are monkeys , there is no conclusion , here middle term bananas which must be distributed once , but in both premise bananas is not distributrd term so $\text{it violate rule 2}.$ so, $\textbf{it is not correct.}$
hence only $\text{B}$ is correct Answer.