ER Diagram translation to tables
Enforcing option (c), using a type attribute:

CREATE TABLE Computer
( computerID INT NOT NULL
, computerType CHAR(1) NOT NULL
, PRIMARY KEY (computerType, computerID)
, CHECK (computerType IN ('D', 'L'))
) ;

CREATE TABLE Laptop
( computerID INT NOT NULL
, computerType CHAR(1) NOT NULL
, battery WHATEVER
, PRIMARY KEY (computerType, computerID)
, CHECK (computerType = 'L')
, FOREIGN KEY (computerType, computerID)
    REFERENCES Computer (computerType, computerID)
) ;

CREATE TABLE Desktop
( computerID INT NOT NULL
, computerType CHAR(1) NOT NULL
, monitor WHATEVER
, PRIMARY KEY (computerType, computerID)
, CHECK (computerType = 'D')
, FOREIGN KEY (computerType, computerID)
    REFERENCES Computer (computerType, computerID)
) ;


and using deferred foreign key constraints on nullable columns:

CREATE TABLE Computer
( computerID INT NOT NULL
, laptopID INT NULL                                    -- these 2 columns 
, desktopID INT NULL                                   -- are NULLable
, PRIMARY KEY (computerID)
, UNIQUE (laptopID)
, UNIQUE (desktopID)
, CHECK ( laptopID IS NOT NULL AND desktopID IS NULL   -- but only one of them 
       OR desktopID IS NOT NULL AND laptopID IS NULL   -- is NULL
        )
, CHECK (laptopID = computerID)                        -- and the non-NULL one is
, CHECK (desktopID = computerID)                       -- equal to computerID
) ;

CREATE TABLE Laptop
( laptopID INT NOT NULL
, battery VARCHAR(20) NOT NULL
, PRIMARY KEY (laptopID)
, FOREIGN KEY (laptopID)
    REFERENCES Computer (computerID)
) ;

CREATE TABLE Desktop
( desktopID INT NOT NULL
, monitor VARCHAR(20) NOT NULL
, PRIMARY KEY (desktopID)
, FOREIGN KEY (desktopID)
    REFERENCES Computer (computerID)
) ;

ALTER TABLE Computer
  ADD CONSTRAINT Laptop_Computer_FK
  FOREIGN KEY (laptopID)
    REFERENCES Laptop (laptopID)
    DEFERRABLE INITIALLY DEFERRED
, ADD CONSTRAINT Desktop_Computer_FK 
  FOREIGN KEY (desktopID)
    REFERENCES Desktop (desktopID)
    DEFERRABLE INITIALLY DEFERRED ;


Tested in Postgres, at SQL-Fiddle