Pulsars a. spin very rapidly when they're young. b. are the cause of gamma-ray bursts. c. spin very slowly when they're young, and gradually spin faster as they age. d. generally form from 25-solar-mass stars. e. emit radio in all directions.

A. spin very rapidly when they're young.

Explanation:

A pulsar (pulsating star) is a neutron star that emits very intense jets of electromagnetic radiation in the range of radio waves, X-rays or gamma rays, at short and periodic intervals due to its intense magnetic field that induces this emission.  

One of its main charateristics is its  rapidly rotation (spin) when this star is young. So, as the star spins, beams of radio radiation from it sweep through space. If one of these beams points toward the Earth, we observe a pulse.  

This is possible when the magnetic pole of the star "points" to the Earth and then stops pointing a thousandth of a second later due to the fast rotation of the star, appearing again when the same pole returns to point towards Earth.