Which symbol can be used to indicate a chemical reaction that takes place in the presence of a catalyst? a: 350° c b: pt c: 0.5 atm d: the triangle

The correct answer is option B i.e. Pt

Following are the reason for identifying it as a correct answer. 

Option A reflects the temperature at which reaction has been performed. It doesn't signify use of catalyst.
Option B signifies that, Pt is employed as a foreign material in the reaction system. The role of this foreign material is to accelerate the reaction rate. Such material is referred as catalyst. With reference to catalyst, it must be noted that, Pt is a heterogeneous catalyst. 
Option C signifies the pressure of reactant gas, at which reaction is carried out. atm is the unit of pressure of reactant gas. 
Option D is the symbol that is very often used to depict that reaction is performed at higher temperature. 

Thus, among all the available options, option B i.e. Pt is a correct answer 

The symbol that can be used to indicate a chemical reaction that takes place in the presence of a catalyst is letter B: Pt. Platinum is the metal which is often used as a heterogeneous catalysts for several reactions. Catalyst lowers the activation barrier of reaction, thereby accelerating the reactions rates.