Elastomer identification: which of the following is an elastomer (rubbery polymer)?

Introduction / Context:
Elastomers display large reversible deformations. Among the listed materials, only polysulfide rubber (Thiokol) is an elastomer; the others are rigid thermosets or thermoplastics.

Concept / Approach:
Phenolic and urea–formaldehyde are thermosets; polystyrene is a brittle thermoplastic at room temperature; Thiokol is a synthetic elastomer with notable fuel and solvent resistance.

Final Answer:
Thiokol (polysulfide rubber)