Increasing order of electronegativity is:
Si < P < S < Cl
Electronegativity may be defined as the tendency of an atom to attract shared pair of electrons towards itself in covalently bonded molecules.
Electronegativity changes with respect to the atomic radius and effective nuclear charge as follows:
Atomic Radius: The greater the size of the atom, the lesser the electronegativity, since the electrons being farther away from the nucleus, experience a lesser force of attraction.
Effective Nuclear Charge: The greater the nuclear charge, greater will be the electronegativity because the increase in nuclear charge causes electron attraction with a greater force.
Since the nuclear charge increases due to an increase in atomic number, electronegativity increases from left to right in a period.
Therefore,
Si < P < S < Cl (Electronegativity)