Enzyme inhibition

A competitive inhibitor increases the $K_m$ of the enzyme: a higher substrate concentration is needed to reach $\frac{1}{2}V_{max}$.
So the statement that it decreases $K_m$ is incorrect.

Malonate resembles succinate and competes with it for the active site of succinic dehydrogenase (competitive inhibition).
Competitive inhibition is reversed by adding excess substrate, so the statement that excess succinate does not reverse it is not correct.

Succinic dehydrogenase is the Krebs cycle enzyme that converts succinate into fumarate.
A competitive inhibitor is a substance that closely resembles the substrate in its molecular structure and competes with it for the active site of the enzyme.
Malonate ($^-OOC-CH_2-COO^-$) is very similar in structure to succinate ($^-OOC-CH_2-CH_2-COO^-$); both are dicarboxylic acids.
Malonate binds to the active site of succinic dehydrogenase but cannot be dehydrogenated, so the substrate cannot bind and the enzyme action declines.
This inhibition can be overcome by increasing the concentration of the substrate (succinate), which is the characteristic feature of competitive inhibition.
$\alpha$-ketoglutarate, malate and oxaloacetate are intermediates of the Krebs cycle, not the classical competitive inhibitor of this enzyme.
So the competitive inhibitor of succinic dehydrogenase is malonate.

A competitive inhibitor closely resembles the substrate in its molecular structure.
It competes with the substrate for the active site of the enzyme; when the inhibitor occupies the active site, the substrate cannot bind and the enzyme action declines.
For example, malonate competes with succinate for the active site of succinic dehydrogenase.
Competitive inhibition can be overcome by increasing the concentration of the substrate.
A non-competitive inhibitor binds at a site other than the active site and changes the shape of the enzyme; adding more substrate does not overcome it.
In the first statement the roles are confused (the substrate does not compete with the enzyme), and the second statement is not a general feature.
So the correct statement is that competitive inhibition is seen when the substrate and the inhibitor compete for the active site on the enzyme.