why is MU, said to be the slope of TU ?????

MU is the change in the utility that a consumer derives by consuming an additional unit of a commodity. Mathematically, it is presented as:

Graphically, the slope of TU curve between the points C and D is 6. The vertical difference depicts the change in the Total Utility, which is 6 (i.e. 24 - 18) and the horizontal difference depicts in the change in the quantity of goods consumed, which is 1 (i.e. 4 minus 3). So, the slope of TU curve between C and D is 6.

Alternatively, we can put it in different words as, the consumer has gained 6 utils of utility just by consuming one additional unit of the commodity (i.e. from 3 units to 4 units). 

Thus, the slope of TU is termed as MU.