As suction pressure in a reciprocating compressor increases, and the HP decreases, compressor capacity:

When the suction pressure in a reciprocating compressor increases, it typically results in an increase in the density of the refrigerant vapor entering the compressor. This greater density allows the compressor to move more refrigerant per unit of time, which effectively increases the compressor's capacity.

The decrease in horsepower (HP) associated with rising suction pressure can occur because the compressor is operating more efficiently under those conditions. As the system pressure rises, the compressor does not have to work as hard to compress the vapor, meaning it requires less energy for the same amount of refrigerant. This relationship highlights the direct correlation between suction pressure and compressor capacity.

In contrast, if suction pressure were to decrease, the capacity would generally decrease as well because less refrigerant mass is being moved through the system. Thus, the answer correctly indicates that an increase in suction pressure results in an increase in compressor capacity.