What did you pay for shares in Stock B? If you paid nothing extra, then your basis is $0.

If some of your shares in Stock A were converted to Stock B, then your basis is what you paid for Stock A.

For every stock spin off, you must allocate basis of the original shares to the new company spun off.  Normally the companies have something on their websites to walk you through how to allocate basis in spin offs as it's based on the price the shares trade at the next day and often complicated.

If you post the companies involved, you could get better assistance.  Do it as a new post though, don't reuse older posts.

First you need to determine the tax status of the spinoff. You allocate basis only if  the tax opinion states that it is a tax-free spinoff.  If it is taxable, your basis is the fair market value at the date of the spinoff of the new shares and there is no allocation of basis from the parent company shares.