Explanation :
The magnetic field due to a solenoid is given as :

where,
is the permeability of the medium.
n is the number of turns or loops in the coil.
I is the current flowing in the solenoid.
The magnetic field due to a solenoid can be increased by increasing the number of turns, current and placing an iron core inside the solenoid.
So, the correct option are:
(1) using more wire loops
(2) using a larger current
Hence, the correct options are (a) and (c).