Q.The catalyst in a chemical reaction provides an alternate pathway or reaction mechanism by decreasing ;

A catalyst speeds up a reaction by providing an alternative pathway with lower activation energy — option (c).

A catalyst speeds a reaction by lowering its activation energy — option (a).

Selectivity is a catalyst's ability to steer a reaction toward one specific product.

Synthesis of ammonia by Haber's process (Fe catalyst) is a heterogeneous catalysis example.

Shape-selective catalysis depends on a catalyst having uniform, molecule-sized pores/cavities — a structural feature unique to zeolites among the given options — so (a) is correct.

What shape-selective catalysis means

Shape-selective catalysis is a reaction whose outcome (which reactants can react, or which product isomer forms) is controlled by the pore size and cage geometry of the catalyst, which physically excludes molecules that are too large to enter, or too large to form and exit. This requires a catalyst with a rigid, uniform, microporous framework.

Why zeolites fit

Zeolites are microporous, three-dimensional aluminosilicates ($M_{x/n}[(AlO_2)_x(SiO_2)_y]\cdot mH_2O$) built from $SiO_4$ and $AlO_4$ tetrahedra sharing oxygen atoms, forming honeycomb-like frameworks with cavities and channels of precise molecular dimensions. Because only molecules of a matching size/shape can diffuse into these channels to reach the active Bronsted-acid sites, zeolites act as molecular sieves and catalyse reactions selectively by size/shape — the classic industrial example being ZSM-5, used to convert alcohols directly into gasoline-range hydrocarbons and to crack larger hydrocarbons in petroleum refining.

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The two classes of catalysis are distinguished purely by whether the catalyst shares the same phase as the reactants (homogeneous) or is present in a different phase (heterogeneous, generally a solid surface).

Homogeneous catalysis

Catalyst and reactants are all in one phase (all gases, or all in solution). Example: in the lead-chamber process, gaseous $NO$ catalyses the oxidation of gaseous $SO_2$: $2SO_2(g)+O_2(g) \xrightarrow{NO(g)} 2SO_3(g)$ — catalyst and reactants are all gaseous.

Heterogeneous catalysis

Catalyst is in a distinct phase from the reactants — most commonly a solid catalyst acting on gaseous or liquid reactants, where the reaction proceeds via adsorption of reactants onto the catalyst's surface. Example: Haber's process, $N_2(g)+3H_2(g)\xrightarrow{Fe(s)}2NH_3(g)$, where solid iron catalyses a gas-phase reaction.

The key difference

Selectivity is the catalyst's ability to steer a reaction so that it forms mainly one particular product among several that could otherwise form.

Different catalysts acting on the same reactants can favour different reaction pathways, so the choice of catalyst can determine which product is obtained preferentially. For example, in the reaction of CO and H$_2$: with a Cu/ZnO-Cr$_2$O$_3$ catalyst, methanol (CH$_3$OH) is obtained; with a Ni catalyst, methane (CH$_4$) is obtained; and with different conditions/catalysts a mixture of higher hydrocarbons can result. This property of a catalyst — choosing/directing the reaction to …