A radioactive substance emits:

The correct option is B

$\mathrm{\alpha }$-radiations or $\mathrm{\beta }$-radiations

The explanation for the correct option

B. $\mathrm{\alpha }$-radiations or $\mathrm{\beta }$-radiations

1. The radiation is released from the nucleus of the atoms.
2. So, a radioactive substance emits: $\mathrm{\alpha }$-radiations or $\mathrm{\beta }$-radiations.
3. Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus.
4. A beta particle, also called beta ray or beta radiation, is a high-energy, high-speed electron or positron emitted by the radioactive decay of an atomic nucleus during the process of beta decay.
5. There are two forms of beta decay, β⁻ decay and β⁺ decay, which produce electrons and positrons respectively.

Thus, option B is correct.

The explanation for the incorrect options

1. The gamma rays are emitted by radioactive elements but these rays are not emitted simultaneously with alpha and beta rays.
2. Gamma-rays do not exist in the form of particles.

Thus, option A and option C are incorrect.

1. x-rays are not emitted from the nucleus.

Thus, option D is incorrect.