d
\(\begin{array}{l} Given,\theta  = {45^ \circ },r = 0.4m,g = 10m/{s^2}\\ T\,\sin \,\theta  = \frac{{m{v^2}}}{r}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( i \right)\\ T\,\cos \,\theta  = mg\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,....\left( {ii} \right)\\ From\,equation\,\left( i \right) and \left( {ii} \right)\\ We\,have,\,\tan \theta  = \frac{{{v^2}}}{{rg}}\\ {V^2} = rg\,\,\,\,\,\,\,\,\,\,\,\,\theta  = {45^ \circ }\\ Hence,\,speed\,of\,the\,pendulum\,in\,its\,circular\\ path,\\ V = \sqrt {rg}  = \sqrt {0.4 \times 10}  = 2m/s \end{array}\)