According to Keynes, the first fundamental postulate of the classical theory of employment
I. The wage is equal to the marginal product of labour
or
$$
W(N) = P \cdot \frac{dQ}{dN}(N)
$$
where $W(N)$ is a money wage, $N$ is an employment level, $Q(N)$ is a physical productivity of labour at that employment level and $P$ is a price of goods or as we call it today price level.
In accordance with the principle of diminishing returns $\frac{dQ}{dN}(N)$ is a decreasing function of an employment level $N$.
$W(N)$ is a non-decreasing function for those who agree to work for a smaller wage are hired first.
Then the money wage $W(N)$ is equal to $P_w \cdot W_r(N)$, where $P_w$ is a price of wage-goods (or as we call it today consumer or final goods) and $W_r(N)$ is a real wage. Keynes uses it in the second postulate of classical theory:
II. The utility of the wage when a given volume of labour is employed is equal to the marginal disutility of that amount of employment.
That is to say, the real wage of an employed person is that which is just sufficient (in the estimation of the employed persons themselves) to induce the volume of labour ctually employed to be forthcoming
Thus for a production of wage-goods the first postulate can be written as $$ P_w \cdot W_r(N) = P_w \cdot \frac{dQ_w}{dN}(N)$$
or
$$ W_r(N) = \frac{dQ_w}{dN}(N)\tag{1}\label{wg}$$
where $Q_w(N)$ is a physical production of wage-goods.
For non-wage-goods (or as we call them today investment goods) the equation will be different:
$$W_r(N) = \frac{P_{n-w}}{P_{w}}\frac{dQ_{n-w}}{dN}(N)\tag{2}\label{nwg}$$
where $P_{n-w}$ and $P_{w}$ are prices of non-wage-goods and wage-goods respectively.
Essentially we can't "pay" employees in non-wage-goods!
Now we can comprehend three out of "four possible means of increasing employment:"
(b) a decrease in the marginal disutility of labour, as expressed by the real wage for which additional labour is available, so as to diminish 'voluntary' unemployment;
i.e. $W_r(N)$ in equations $\eqref{wg}$ and $\eqref{nwg}$ is shifts down so intersection with the function on the right side of equations shifts to the right.
(c) an increase in the marginal physical productivity of labour in the wage-goods industries
i.e. the function on the right side of $\eqref{wg}$ shifts up so intersection with the function on the left side of the equation shifts to the right.
And finally:
(d) an increase in the price of non-wage-goods compared with the price of wage-goods
i.e. increase of the ratio $\frac{P_{n-w}}{P_{w}}$ in $\eqref{nwg}$ effectively shift the marginal product of labour for non-wage goods expressed in wage-goods $\frac{P_{n-w}}{P_{w}}\frac{dQ_{n-w}}{dN}$ up so the intersection with $W_r(N)$ will move again to the right.
Please note that Keynes aims to refute (some of) the above speculations.
