b
\(\overrightarrow F = i(\,\overrightarrow {l\,} \times \overrightarrow B )\) \(==>\) \(\overrightarrow F = i\,[l\,\hat i \times {B_0}(\hat i + \hat j + \hat k)] = {B_0}il[\hat i \times (\hat i + \hat j + \hat k)]\) \(==>\) \(\overrightarrow F = {B_0}il[\hat i \times \hat i + \hat i \times \hat j + \hat i \times \hat k]\)\( = {B_0}il[\hat k - \hat j]\)
\(\{ \hat i \times \hat i = 0,\,\hat i \times \hat j = \hat k,\,\hat i \times \hat k = - \hat j\} \)
\(F = \sqrt 2 {B_0}il\)